I have been pondering over that question in this and other sites. It is a tricky and personal subject.
In a perfect Windows 7 forum, we would only be dealing with the latest release, so that helpers can speak with more authority and associated knowledge with the OS in question. That perfection is not about to happen. There are numerous requests, also here for example, for computer details, before help is offered. By the same token, it is becoming important to know what release we are dealing with. Axiomatically, early builds were buggy. In the vast majority of cases, the use of, and reference to them on these sites, were irrelevant, as the clearance of the bugs was constantly ongoing.
It is often easy to spot if a member is using such a build, but sometimes, after a drawn out thread, it is not so transparent. I normally, right or wrong, ignore help requests for older builds, but I am sure the help is still forthcoming from others.
To ask members to say what build they are using (or even put it in their CP's) would be impertinent, however important.
But, ref you OP, how would you define, appropos my words, which questions have remained unanswered? It would need an astronomical amount of searching on the part of the Administrator or moderators. Perhaps, as you suggest, a cut-off period could be applied, now, but on a temporary basis. Keeping in mind that the RC is still being used, quite legally, for some months yet, maybe a cutoff period from before that was released would be appropriate?
It would be interesting to read the views of moderators and the more prolific posters.