FWIW, I think obsolescent is too obscure of a word, I just mentioned it because it’s used by another large/old language in this context, but I personally think “obsolete” would be fine (and obviously we don’t have to take over the wording from the C standard, but can come up with something appropriate for us).
Its definition doesn’t even imply removal; obsolete:
- out of use or practice; not current
- out of date; unfashionable or outmoded
Personally, I think that it’s a more accurate word than discouraged, but that would also work.