Is there a non-corporate alternative to GitHub?

Any Linux server with sshd installed is a git server if you want. There is even a bundled gitweb front end (some perl dependencies) if you need it. No other software needed

I've ran that set up for many years with no need for GitHub/gitlab features

Gitlab, Sourcehut, Gitea, or Gogs? Or cgit?

why is gitlab non-corporate?

It can be self hosted

You might like https://codeberg.org/.

Sourcehut. https://sr.ht

https://libreplanet.org/wiki/FSF_forge_evaluation

Lots of FOSS projects run their own places, e.g. debian salsa, and of course you can run git anywhere. I guess you are mostly refering to the showcase/discovery aspect of github. For that many people just use github or gitlab for the visibility.

I am not aware of a "brandname" FOSS-only place, but apparently the fsf wants to change that.

You haven't checked out GitLab?

Stand up your own git server.

the only problem is that an own git server makes it difficult to contribute. but there are people working on that using activitypub.

What do you mean by non corporate?

501c3s (nonprofits) are a form of corporation.