Originally posted by Munin:

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.

I have never understood why Iron Gate chose to restrict it to Discord.
Not everybody likes Discord or is able and willing to create a Discord account. I certainly do not and I quit using Discord years ago. I'll never go back.
Many developers (including Coffee Stain) use the readily available system for a test branch via Steam.