[2021] access to public-test and/or unstable branch?

Yes, there is a test branch available, but it's private (more or less).

To get access you need to join our Discord, go to #rules-and-info, react with "V" (:Valheim:). Then go to the channel #valheim-public-test and read the pinned messages on how to activate the test branch. You can also provide feedback and discuss changes in that channel.

Iron Gate also uses the test branch option via Steam. But it is a private test branch which requires a password. The only way to get access is how I described, after that you could theoretically deinstall Discord again.