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In context I read that as database client, meaning the application server (which is a client to the database) providing the service to the user. Having that be able to generate IDs could be useful when needing to refer to the same entity, even if there is data that has to exist in some separate database for some reason.

That is indeed what I had in mind, although I did leave it intentionally vague as everyone can asses what’s best for their own situation

I’ve once attempted to implement a solution where ids are generated by UUIDv5 from a certain owner and the relationship of the new item to the owner; that way, users cannot generate arbitrary ids but can still predict ahead of time their new ids to ease optimistic behaviour.

I think if you have (tenant, uuid) or (user, uuid) or whatever as your primary key, that's fine. If that tenant or user or whatever generates rubbish keys, that's their problem.