Unless you have a specific reason not to do so, it’s recommended to use the tests-passed branch of Discourse. See further:
Furthermore, you can assume that plugin or customisation with the experimental tag (as the ActivityPub plugin doees) only works reliably on the latest version of Discourse.
More specifically to your implicit questions about version management, in Discourse plugins this handled via the .discourse-compatibility file which you can read about here:
See the ActivityPub plugin’s compatibility file here:
As the ActivityPub plugin is still experimental, ensuring backwards compatibility has not been a focus, nevertheless I will investigate adding explicit 3.3.2 support to that file early next week.
But as mentioned above, unless you have a good reason not to, I’d suggest using tests-passed.