it is completely fine, so long as the other requirements for a quality answer are met.
However, just because it should be allowed, do not mean it is going to be a good answer. Answering with it is maybe unlikely to earn upvotes, and people who think that it is bad, will as usual, downvote.
To be a good answer, it should follow the usual requirements.
Matching off against the question.
Talking from personal experience etc.
In this case, the personal experience should include how long it took to receive access after making a beta request.
The answer should also be clearly showing amongst the basic facts, that it is in Beta and what sort of arrangement the beta is. Whether it is a Open-Beta, a Email-in where everyone is accepted, a email-in where it is open only to randomly selected or the first X requester. It also should specify other information, such as whether it free only on beta and become paid after etc.
But there is no reason not to allow it.
- we don't excude beta software in general.
- we don't exclude paid software.
- we don't exclude software which you have to go to a physical shop to buy
- or even that is only available second hand.
There are many reasons why it might be a poor solution.
But that is for voting, not for ruling on meta.
Unless the specific case should be deleted as "Very Low Quality".
Eg if the OP specified things like "Mature Product", "Industry Standard for X" as a important requirement.