I think one possible limitation here is that the AskLibO site is powered by AskBot, which is a completely separate piece of software that has nothing to do with LibreOffice (it's open source, though), and activity in that code base seems to have been rather low lately. So I'm not sure how feasible something like adding a custom text box for that information would be.
Though regardless, I would be adverse to making something like operating system / version numbers mandatory because not all questions on this site can be shoehorned into the format of a bug report. There are lots of meta questions etc. (e.g. this one) which such details wouldn't apply.
Insufficient information in questions seems to be a rather common occurrence on Q&A sites, though, and Stack Overflow has tried various things to try and improve question quality. Maybe some ideas could be pulled from there?
One change I would like to see, and one that I feel like would be an easy low-hanging fruit improvement, is just adding more instructions below the "Please ask your question here" text. Y'know, for the "just in time" theory of user behaviour and whatnot.
Right now it just says:
Maybe we could add some more bullet points here? e.g.
I think the [meta] tag would also be a good fit for this question.
Your idea sounds spot on at first, but @ahiijny brought up good points, since many questions aren't OS or LO version specific (like "How do I use the SUM function in Calc?"). No CMS is perfect, but I mostly like this site's format. Browsing the older style Open Office forum sends shivers down my spine in comparison (lots of useful info there too though). Intelligent category pages or a proper home page instead of direct into question list might be good. Tag pages are ok, but not always the best.
I think you need to sign up here and post this suggestion there, as that is the place that makes this software we all use.