What is your bar for “useful”? Let’s start there and we’ll see what evidence can be offered.

User count? Domain? Scope of development?

You have something in mind, obviously.

If you're asking me to define a very clear bar, it's obvious nothing cleared it.

Anything that proves that LLMs increase software quality. Any software built with an LLM that is actually in production, survives maintenance, doesn't have 100 CVEs, that people actually use.