Parmandur
Book-Friend, he/him
@Remathilis did a breakdown somewhere recently. It's actually not that bad: of the 13 5E Classes, all but two have four Subclass Slots, all within spitting distance of the OneD&D progression: so a compatibility sidebar (which they stated was something that would be provided as the rules get nailed down in the last packet) build down to "take the Subclass features at these points instead" and a few other details, Mayne.Yeah that's going to be a very rough kind of compatibility at most. Like, if you create a 5E character and use Tasha's and Xanathars options, and play them in a 1D&D game, you'll probably be fine, but trying to jam a subclass from either of those on to a 1D&D character is unlikely to work out well (esp. from certain classes - IIRC only Barbarians match the 1D&D subclass progression exactly, and some are really big mismatches - I saw a big table somewhere, maybe on the 1D&D reddit?).
The two outliers are Bard and Fighter. Bard has 3 Subclass features, not 4, ao might need a bonus Feat or something. The Fighter has 5 Subclass feature Levels, so it will be interesting to look at the Fighter UA to see what can be done.