Can FLOSS exist under non-FLOSS ownership, and if not, can FLOSS exist on a platform under non-FLOSS corporate ownership?

The specific use case in mind is FLOSS software on GitHub. To me, Microsoft buying GitHub invalidated it as a platform for hosting my FLOSS repositories, so I moved to notabug.org.

GitLab is a tricky platform as well, since it isn't fully open source, but it seems to have been declared good enough by many who left GitHub.