I don’t think the addon maker is in the wrong. The version that is meant for the live version of the game appears to be working and free and the author only solicits donations on his patreon page (even in the github issue).
It says in the addon rules thread:
“[…] however, such requests [for donations] should be limited to the add-on website or distribution site and should not appear in the game.”
I seriously don’t think that the addon developer should be obliged to provide a version for a beta of the game, especially since during the ongoing state of development that the beta is in you can’t possibly expect the author to keep up with development on live servers and beta version.
There is a version of the addon that you can download for free that works on live servers. That’s what the author provides to the public.
That there’s a preview version that only pledges have access to is a grey area at best to me. The pledge does not offer access to extended features or premium content (addons like Zygor and TSM do, even for their live versions, and Blizzard does not bother them either), it offers access to a preview version for a game not yet released.
I would assume that as soon as Shadowlands goes live, the versions on Curse and other addon repositories would be updated to work with the live version at no charge, but right now the beta is limited to a small pool of testers in relation to the entirety of the WoW userbase and I would not assume that this case falls under Blizzard’s addon rule.