There is this document: https://wiki.osarch.org/index.php?title=BlenderBIM_Add-on/BlenderBIM_Add-on_Roadmap though the "The current roadmap" is not current anymore. However the intentions of the three phases of alpha / beta / stable (0.0.x, 0.x.x, x.x.x) are still there.
From the description of beta (i.e. the shift from pure feature development to other aspects like tutorials, docs, usability, fundraising) it seems as though we are clearly entering beta territory. However these seem to me to be secondary criteria. The primary criteria in my mind of what makes us ready for beta is that people are using it productively for commercial-grade work. So far, I am only personally aware of @Ace, Pale-blue-dot, and @theoryshaw who are using it for end-to-end design and documentation (maybe @ChubbyQuark ? ). I don't count, of course. A lot of people are using it for portions of their workflow (i.e. model checking, non-design aspects like 4D/5D, quantity take-off) but I struggle to put a number on "a lot" and it's very hard to know how much it contributes to commercial-grade work. At the end of this year I'll put together a post to share some stats but for example right now we have 50,000 unique IP downloads of the BlenderBIM Add-on per year. That's certainly not a small number.
I think the best way to know whether we're ready for beta is to simply ask people. "Do you think this is beta software, or does it still feel like alpha software?" If enough people say it feels like beta, then it is beta. If they say it still feels like alpha, ask them "which part?" and then we fix that part until it becomes beta.
If 25 people can tell me it feels like beta (for their respective usecases that go beyond just model viewing), then I'd tentatively say we're beta. If 50 people tell me, then I'd confidently say we're beta. So ... as is the core of any open source project: the answer lies in the community.