In the meantime, 8.0.5 and 8.0.6 were released: https://www.mongodb.com/docs/manual/release-notes/8.0/
Those appear to be patch releases for major releases, not rapid releases, so the source should be available: https://www.mongodb.com/docs/manual/reference/versioning/
There is a forum post without an answer: https://www.mongodb.com/community/forums/t/source-code-in-github/316442
Is MongoDB no longer source-available?
We have restored the synchronization, and the latest code updates are now available via GitHub. Thank you for your patience and understanding.