Apparently supabase/auth only supports v4 for the ids. I admit to not knowing the intimate details of the differences but from what I can tell, there isn't a good technical reason for not supporting it now, even though there isn't an official pg18 flavour yet.
Anton Melser suggests that Supabase's auth system should support UUID version 7 for user IDs. He notes that currently only version 4 is supported and questions the technical reasons for this limitation, despite the lack of an official pg18 implementation.