Phoenix already has it live. CLN supports it natively, and LDK is building it in so any wallet using that toolkit gets it eventually. And yes, backwards compatibility is exactly how it works. A wallet can try BOLT12 first and fall back to BOLT11 if the other side doesn't support it. No reason it has to be all or nothing. The real question is when LND adds native support instead of making people bolt on LNDK as a workaround.