Bitcoin Core can only sign messages from legacy bitcoin addresses, in my experience.
Yes, that's correct. Message signing is currently only supported for P2PKH addresses.
With legacy addresses being deprecated in the software, does this mean that Bitcoin Core will no longer be able to sign any messages from the new descriptor wallets verifying someone holds the keys to their bitcoin addresses?
This is conflating two things: legacy wallets and legacy addresses. They're independent. If you want to, you can create/import a descriptor for P2PKH addresses in a descriptor wallet, which can generate P2PKH addresses, for which message signing will work. You can also use a legacy wallet to construct P2SH or P2WPKH addresses, which don't support message signing.
If this is true, is there any work being done on adding message signing functionality for bech32 addresses?
Yes, there is an ongoing though somewhat stalled effort to define and implement a generic message signing standard that would work for any address type, BIP322.