I'd assume that'd be handled by a client, right? Not a new NIP I'd assume

Not a coder but I'd think the logic is either:

A:

UserA posts Note0, posts Note1 as a reply to Note0, posts Note2 as a reply to Note1, etc. Client presents this as a chain/thread.

B:

UserA posts Note0, posts Note1 as a reply to Note0, posts Note2 as a reply to Note0, etc. Client presents this as a chain/thread.

I'd assume A makes more UX sense.