I would like to know if a given latin proverb is still known and used by latin speakers nowadays — most likely in Vatican I guess. Would such a question be on-topic on latin.SE ?
Questions about the contemporary use of Latin are certainly on-topic, but the more specific you can be, the better. For example, if you ask:
Is this proverb still used by contemporary Latin speakers?
Someone could respond by saying, "Why yes, I speak Latin and I just spoke this proverb as I was reading your question, so yes, it is still used." That's probably not what you were going for!
But, you could ask the question in a slightly different way that more reliably indicates what you think of when you think of a proverb being used:
Do any documents published by the Vatican since 1900 use this proverb?
Do any Latin dictionaries or collections of Latin proverbs published since 1950 include this proverb or a variation of it?
Does author X use this proverb in any of his published writings?
In short, the more objectively answerable your question is, the better. At the very least, if asking the general "is this used" question, indicate in the body of your question what type(s) of evidence you are looking for.