MIT core + "ee" (enterprise edition) commercially licensed extension subdirectory isn't that strange of a occurrence nowadays.

I also wouldn't pin it as chat app specific. Quite a few VC funded open core software has adopted that pattern post ~2020(?): cal.com, Dagster, Gitlab

Yea, the license is modeled after the Gitlab license. All of the core chat/RAG/agent logic is fully MIT, and >99% of deployments of Onyx are using the "community edition"!