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So, here's a point that, while it's created a lot of emotional backlash, due to people being devoted to Our Lady ... I feel that the Modernists have done much worse than.  Both in the original "note" and here with Tucho's explanation, it was clear they said that the title COULD be legitimate if properly explained, and we have Traditional Catholics, such as the Dimond Brothers, and some approved pre-Vatican II theologians who have said the same thing.

I believe that both sides do have a point, as the not of what "co-" means can have certain connotations or nuances in modern languages, even if in scholastic terms, it's quite legitimate to use even of a secondary efficient cause, a caused cause that only achieves its effect by virtue of (in the power of) the primary cause, and, as St. Thomas explains, these are often referred to as "concausae" (co-causes).  So, if a Redeemer is the cause of Redemption, then a co-Redeemer is a co-cause of Redemption, or a concause, which is a subordinate/secondary term that does not have the connotation of putting the "co-" on the same order of causality as the primary cause.

So it's legitimate from a scholastic term, but in vernacular languages, it can easily lead to misunderstandings.

So, for instance, if I hurl a rock through a window, in scholastic terms, the rock itself can be a "concausa" of the window's destruction, but we know that any potency that the rock had to break the window was imparted to it by the primary efficient cause, namely myself, and it had no power on its own to break a window, nor would anyone consider it somehow responsible for the destruction.  But in scholastic terms, it can be considered a co-cause.

Similarly, if I use a pen to write a book, in scholastic terms, it could be called a "concausa", a co-cause of the book being written, but of course in English nobody would therefore ever call the pen a "co-author" of the book, since the term "co-author" implies a contribution on the same order of causality as the principle efficient cause.

That's where we're at in terms of Co-Redemptrix.  In English, the term strongly implies (similar to co-author in the example above) that Our Lady was a Redeemer on the same order of causality, and that would be false, even if it's perfectly acceptable to say that in scholastic/theological terms, and the term would have to be accompanied by explanation.

Now, while normally the reason the Conciliar Modernists have used to eschew the term might be legitimate on its own, there's every reason for skepticism regarding their true motives.  Then, even if they are sincerely stating their motives, they are largely culpable anyway due to the fact that it's certainly the case that the abysmal catechesis (or, rather, lack thereof) by the Conciliar would make it nearly impossible to explain the term to the average Novus Ordite who could not pass a quiz based on Baltimore No 1.  80%+ of them undoubtedly still believe that the Immaculate Conception refers to the conception of Our Lord rather than that of Our Lady.

But even then one might rightly be cynical of their motives of wanting to spare the faithful of "confusion".  Bergoglio positively reveled in and bragged about causing chaos and "messes", and were they concerned about causing confusion with "Amoris Laetitia" and "Fiducia Supplicans".  With the latter, Bergs provided explanations for the popesplainers to deploy in justifying it, so why couldn't they have done the same with this?  Reminds me of when Ratzinger wanted to fix the bad vernacular translations of the NOM, where the US bishops pushed it back for a couple years due to the "confusion" it would cause ... to say "for you and for many" instead of "for you and for all" (great upheaval) ... but then didn't give two hoots to the mass confusion caused by the NOM in the first place, where millions left the Church as a result of that confusion.

Now, it is true that affirming the title would make the Conciliar laity even more prone to being picked off by Prots, who would jump all over it and say, "See, we TOLD you the Catholics worship Mary."  At that point, most Novus Ordites would just say, "yeah, I guess you're right" ... and not "well, by the term co-Redemptrix, we're referring to a secondary co-causation, where the co-Redemptrix is a caused co-cause of the effect created by the primary cause, the Redeemer, having herself been redeemed."  Yeah ... good luck with that.