PostHog has always been open core and source-available. What's new is that they now publish a GitHub repo containing just the open source parts.

Has it? This is from that repo, "PostHog FOSS is a read-only mirror of PostHog, with all proprietary code removed." Seems they have some proprietary code in the non-open-source version.

I thought the same thing. What changed, if anything?

Exactly, I was thinking this too!