It seems fairly clear that older books will still be usable, but they will require adaptation. We do not know the full extent of the adaptation required, because we don't know the full extent of the changes yet--there's much still to be learned. However, from the UA doc, we can generally derive the following conclusions:
1. Races from outdated books will still be mostly usable, but you should convert Short Rest abilities to a PB/LR schedule. Further, you get your choice of racial stat bonuses or background bonuses, not both.
2. All backgrounds from outdated books should get a 1st Level (D1D) feat. They should probably include a language as well (if they don't already.)
3. Players should get their choice of either keeping their existing spell list, or getting the spell list associated with their class (Arcane, Divine, or Primal.)
There will likely be more adjustments. However, one thing I expect will not require much (if any) adaptation will be monsters. Those will likely remain exactly the same. Same with XP amounts and levelling. That, I think, is what they mean when they say "backwards-compatible." That is, you'll be able to use every adventure from before pretty much exactly as-is. It's the chargen stuff that's mostly getting changes.
. So now I tend to cull books I don’t actually use or have a nostalgic reason to keep. I try to keep my books below a hundred.
