In my opinion using anything other than tests-passed only really makes sense if you’re on a managed hosting service or: 1) if you have a team managing your community (i.e. you’re a medium to large enterprise); and 2) you have a specific reason for not being on tests-passed, for example you have a multiple customisations that may break.
I would say that both conditions are necessary, because unless you have a team managing your community in the multiple fragile customization scenario your issue is not your branch use but your overall site management (i.e. it’s not sustainable).
Even if both are true, there would be other things to take into account first such as your update policy.
I think the issue is that if you said something like “You can use stable or tests-passed” some people would stick stable in there because it sounds “sensible” when they probably shouldn’t be using it.
Whither beta?
To further the case against the beta branch, in various written and spoken contexts the main confusions it produces are:
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People normally associate the term “beta” with something more bleeding edge than the “standard”. Not the case here.
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Some enterprises consider using it because it seems a bit less bleeding edge than tests-passed and a bit more up-to-date than stable, i.e. again, it seems “sensible”. But in most cases it’s not a good idea.
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“beta” is a term used in both Discourse version numbers and as a Discourse branch name. I’ve found this confuses some people.