Very soon Tor will implement proposal 224, also known as next generation onion services.

This means the addresses will be bigger, whole keys (ed25519) base 32 encoded and with .onion at the end.
Example: a1uik0w1gmfq3i5ievxdm9ceu27e88g6o7pe0rffdw9jmntwkdsd.onion

This means ADD_ONION:BEST will produce a new key (best which is ed25519) rather than RSA1024.

Bitcoin Core should fully support the new address types, or change torcontrol.cpp#L473 from ADD_ONION:BEST to ADD_ONION:RSA1024 . The 2 address types (RSA1024 and ed25519) will work simultaneously for a transition period, which means Bitcoin Core also has to support both to ensure working configurations do not break.

This affects new setups, nodes that previously generated a RSA1024 key and upgrade Tor should be able to preserve that key based on current code.