I haven't been on this stack very long but there seems to be a consistent issue of people not listing their RDBMS platform. Makes answers difficult or irrelevant if submitted. Seems like we need some hand-holding here!
Not all questions are platform-specific, and not all questions are specific to just one platform. Example from today: Which DBMS are more standard compliant? - you only get 5 tags, which 5 platforms would you pick? (Whether it is actually on-topic or not is a different issue.)
You also get all kinds of people who tag questions with MySQL or SQL Server when they meant the opposite, and there are always questions where platform is already obvious, but we need to ask for version specifics before we can answer.
If someone asks a question and you need to know what platform they're using, that doesn't make it hard to answer the question, it just means you need to ask for clarification first.
FWIW, as an answerer, I only follow the tags for the platforms I care about. If someone doesn't bother tagging with the right platform, I don't see it. shrug That doesn't make anything harder for me.
As a moderator, though, I just don't see it all that much. There was one question yesterday where the platform was left off, and I just corrected it. It was easy enough to infer from the code, the error message, and the screen shot, that the platform was SQL Server. The question was answered correctly even without that knowledge, since often both the question and the answer can apply to multiple platforms.
What will happen if you force a platform to be selected is (a) you'll have people go elsewhere because they don't believe they can ask a platform-agnostic question, or (b) people will just pick any or the wrong platform to get past the validation, not realizing that it is important (or not understanding that MySQL is not SQL Server). So, no, I am against forcing a platform to be selected.