I really thought Apple had added a free option (maybe even specifically for open source projects?), but I can't find it anywhere. Can anyone else find that info, or am I misremembering?
Thank you! I'd confused it with recent changes to the iOS program allowing people to self-sign so they can test on their own iOS devices.
It looks like Educational memberships are Sign-In With Apple ID only, which doesn't provide a Developer ID (which is required for code signing, as far as I can tell).
https://developer.apple.com/support/compare-memberships/
What's not clear to me, however, is if the educational option allows for free developer-level memberships.