So ... I'm not entirely sure that the OP is accurate.  While it was universal Church law still, starting in the 1940s and 1950s, Pope Pius XII did start delegating permission to the bishops of various countries to establish standard for the countries.  This came out in the history of the various vigils, where the US bishops had made certain vigils non-fast/non-abstinence days, but it was for the entire country by agreement of the US bishops, and not diocese by diocese.  So I believe the truth is somewhere in between the OP.  I do believe it was for the entire US, not just diocese by diocese, but we'd have to confirm.  This was a slight step in the direction of the national bishops' conferences, but for things like this a national policy does make sense, especially when so many Catholics travelled for Thanksgiving, wondering whether they're bound by the edict of the bishop in the place they were at or the one in their home diocese.

Now, there's also this myth about how the dispensation was granted due to lack of refrigeration during that time, but by 1950, over 80% of US households had refrigerators, and I believe that it would have been much lower than that by the mid-1950s.

I'm just observing Friday abstinence as usual.  I made my stuffing without meat content on purpose, so I could have it today with some cranberry sauce and a meatless patty and maybe some cottage cheese.  Good enough meal for anyone.

So, that's good for you, but the question is about Church law.  There's nothing stopping any individual from abstaining 7 days per week.  I'm sure many people just abstain anyway on this Friday, and I do myself, just because ... why not? ... and I don't like turkey all that much anyway, etc. ... but the question is about whether Traditional Catholics would in conscience be required to do so.

I am (some flavor of) SV myself, but I would have to say it's a massive stretch to say that the US bishops in 1966 did not have the authority to remove the obligation ... so my opinion is that it's not obligatory.  AND, of course, if you're a probablist or semi-probablist (like St. Alphonsus), if there's some reasonable objective doubt about whether a law is binding, it's not strictly binding on any individual conscience.

So, doing a bit or research, Pius XII started the permission for any bishop to dispense, in 1957, and this was renewed in 1962.  But then in 1966 the US bishops removed it for the entire country.

So ... even IF you're an SV, since the permission had been granted by Pius XII, and most of the US bishops in 1966 had been appointed by Pius XII ... unless you're a full-blown ecclesia-vacantist type where every bishops was deposed by 1966 due to not having rejected Vatican II, then that was a legitimate dispensation at that time for the entire country.

Just being a sedevacantist would not suffices to say this was illegitimate, since, the bishops received authority originally from Pius XII, but then the bishops extended it to the entire country in 1966, and so you'd have to hold these bishops to all have been out of office (not just Montini) to hold that they did not have the authority to do it.

1965 is a legitimate reason to disregard anything from the top down starting then. Widespread radical change was indication it was from the top.

Just look what happened immediately to the vowed sisterhood; it was like an atomic bomb:

I treat it simply as a matter of private devotion, as well as reasoning that we must do penance during our lives, and you cannot go wrong with doing what the Church prescribes as a general rule.  If others come to different conclusions, I have no quarrel with them --- as you point out, it's quite tendentious to maintain that the US bishops lacked the authority in 1966 to dispense from this (bad idea, yes, ultra vires, no) --- except to remind the Novus Ordo types, who generally never abstain on Fridays outside of Lent, that at the end of our lives, whether we did penance or not will matter.  The more the better.

The Church mandates these penances so that we do a bare minimum required during our lives.

Loss of office for heresy, as expressed in Canon 188 §4, is not tendentious.