St. Thomas distinguishes the ceremonial precepts and judicial precepts of the Old Law. He says the ceremonial precepts have been abolished at Christ's coming (I-II q. 103 a. 3) and for a Catholic to observe them is a mortal sin (I-II q. 103 a. 4). Regarding juridical precepts, he says (I-II q. 104 a. 3 c.), contrasting and comparing them with the ceremonial precepts:

The judicial precepts did not bind for ever, but were annulled by the coming of Christ: yet not in the same way as the ceremonial precepts. For the ceremonial precepts were annulled so far as to be not only "dead," but also deadly to those who observe them since the coming of Christ, especially since the promulgation of the Gospel. On the other hand, the judicial precepts are dead indeed, because they have no binding force: but they are not deadly. For if a sovereign were to order these judicial precepts to be observed in his kingdom, he would not sin: unless perchance they were observed, or ordered to be observed, as though they derived their binding force through being institutions of the Old Law: for it would be a deadly sin to intend to observe them thus.

The reason for this difference may be gathered from what has been said above (Article [2]). For it has been stated that the ceremonial precepts are figurative primarily and in themselves, as being instituted chiefly for the purpose of foreshadowing the mysteries of Christ to come. On the other hand, the judicial precepts were not instituted that they might be figures, but that they might shape the state of that people who were directed to Christ. Consequently, when the state of that people changed with the coming of Christ, the judicial precepts lost their binding force: for the Law was a pedagogue, leading men to Christ, as stated in Gal. 3:24. Since, however, these judicial precepts are instituted, not for the purpose of being figures, but for the performance of certain deeds, the observance thereof is not prejudicial to the truth of faith. But the intention of observing them, as though one were bound by the Law, is prejudicial to the truth of faith: because it would follow that the former state of the people still lasts, and that Christ has not yet come.

Thus it's clear:
• To observe the Old Law's ceremonial precepts today is always a grave sin.
• To observe the Old Law's juridical precepts is sinful only if they are observed "as though they derived their binding force through being institutions of the Old Law". (St. Thomas realizes that natural law is contained in the Old Law; it is not sinful to observe the natural law. It is sinful to observe it as though you were observing the Old Law.)

I think what needs clarification is what "covenant" is. The OED says (source):

Scripture. Applied esp. to an engagement entered into by the Divine Being with some other being or persons.

[The Hebrew word bĕrīth is also the ordinary term for a contract, agreement, alliance, or league between men. It is constantly rendered in the Septuagint by διαθήκη ‘disposition, distribution, arrangement’, which occurs in Aristophanes in the sense ‘convention, arrangement between parties’, but usually in classical Greek meant ‘disposition by will, testament’. Accordingly, the Old Latin translation of the Bible (Itala) appears to have uniformly rendered διαθήκη by testamentum, while Jerome translated the Hebrew by foedus and pactum indifferently. Hence, in the Vulgate, the Old Testament has the old rendering testamentum in the (Gallican) Psalter, but Jerome's renderings foedus, pactum elsewhere; the New Testament has always testamentum. In English Wyclif strictly followed the Vulgate, rendering foedus, pactum, by boond, covenaunt, rather indiscriminately, testamentum in the Psalter and New Testament always by testament. So the versions of Rheims and Douay. The 16th cent. English versions at length used covenant entirely in Old Testament (including the Psalter), and Tyndale introduced it into 6 places in the New Testament. These the Geneva extended to 23, and the Bible of 1611 to 22 (in 2 of which Gen. had testament), leaving testament in 14 (in 3 of which Gen. had covenant). The Revised Version of 1881 has substituted covenant in 12 of these, leaving testament in 2 only (Hebrew ix. 16, 17).]

Thus bĕrīth, διαθήκη, fœdus (pactum), covenant are applied to God's engagement with Noah and his posterity, Gen. vi. 18, ix. 9–17; to that made with Abraham and his posterity, Gen. xvii, of which the token was circuмcision; to the institution of the Mosaic Law, Exod. xxiv. 7, 8, and to that law or its observance itself, whence the expressions book of the covenant (i.e. of the law), ark of the covenant, blood of the covenant (i.e. of beasts ritually sacrificed), land of the covenant (= promised land, Canaan). The covenant with the Israelites, in its various phases, is commonly called the Old Covenant, in contrast to which the prophets made promise of a new covenant, Jer. xxxi. 31; and this name καινὴ διαθήκη (New Covenant or testament) was, according to St. Luke xxii. 20, applied by Jesus to the new relation to man which God had established in Him. In this sense it is also used by St. Paul and the writer of the Epistle to the Hebrews, who contrast these two covenants (Gal. iv. 24, Hebrew viii. 13, ix. 15, etc.), also called by commentators the Temporal and the Eternal Covenant (cf. Hebrew xiii. 20).]

So, it's clear that to say the Old Covenant is still in force implies that even the ceremonial precepts are still in force for the Jєωs. It also shows that "covenant" signifies "the institution of the Mosaic Law, Exod. xxiv. 7, 8." To claim the Mosaic Law is still in force is condemned by Heb. 8:13: "Now in saying a new, he hath made the former old. And that which decayeth and groweth old, is near its end." And by Pope Eugene IV:

The sacrosanct Roman Church ... firmly believes, professes, and teaches that the matter pertaining to the Old Testament, of the Mosaic law, which are divided into ceremonies, sacred rites, sacrifices, and sacraments, because they were established to signify something in the future, although they were suited to the divine worship at that time, after Our Lord’s coming had been signified by them, ceased, and the sacraments of the New Testament began; ... All, therefore, who after that time observe circuмcision and the Sabbath and the other requirements of the law, it (the Roman Church) declares alien to the Christian faith and not in the least fit to participate in eternal salvation, unless someday they recover from these errors.

Fr. Paul Kramer gives more citations, such as Pope Benedict XIV's Ex Quo:

61. The first consideration is that the ceremonies of the Mosaic Law were abrogated by the coming of Christ and that they can no longer be observed without sin after the promulgation of the Gospel. Since, then, the distinction made by the old Law between clean and unclean foods belongs to the ceremonial precepts, it may justly be affirmed that such a distinction no longer exists and ought not be insisted on. It is true that I the holy Apostles forbade the faithful to eat blood or the flesh of animals which had been strangled. This view was expressed by James at the Council of Jerusalem: "Therefore I judge that those of the Gentiles who turn to God should not be disturbed, but that we should write to them to abstain from what has been sacrificed to idols, from unchastity, from the meat of strangled animals, and from blood" (Acts 15). But it is clear that this was ordained to remove all occasion of disagreement between Jєωιѕн and Gentile converts to Christ. Since this reason has long since vanished, its consequence should also be said to have vanished. "Similarly, we profess that the legalities of the Old Testament, the ceremonies of the Mosaic Law, the rites, sacrifices, and sacraments have ceased at the coming of Our Lord Jesus Christ; they cannot be observed without sin after the promulgation of the Gospel. The distinction of clean and unclean foods found in the old Law pertains to the ceremonies which have passed away with the rise of the Gospel. The Apostles' prohibition on food offered to idols, blood, and the meat of strangled animals was suitable at that time to remove cause for disagreement between Jєωs and Gentiles; but since the reason for this prohibition has ceased to be, the prohibition too has come to an end."

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And so, why is it heretical to say, "The Old Covenant is still in effect, for Jєωs (or for anyone else)?"  Where is this "heresy" explained in your post?


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Here:

So, it's clear that to say the Old Covenant is still in force implies that even the ceremonial precepts are still in force for the Jєωs.