We've always published alpha releases, though we haven't advertised them as GitHub Releases until recently - a blip as we change our release pipeline.
Thanks for the note on the difficulty of finding the last release. GitHub has a custom URL that shows the latest "non-prerelease" release, though I don't see it surfaced anywhere:
https://github.com/pulumi/pulumi/releases/latest
We'll clean up the alphas in the meantime.