The current version is at https://github.com/jsoftware/jsource

There is a relatively recent episode of the ArrayCast array language podcast with Henry Rich who is the primary maintainer and developer of J, talking about new features: https://www.arraycast.com/episodes/episode104-j96

Click on the show notes and there are links to previous episodes with him, talking about the J engine implementation, updates in J 9.03, threading and faster bignum calculations in J 9.04, updates in J 9.5, his views on tacit programming, etc.