That's a great question! Ahem.

I remember being advised to do this ~20 years ago when I was going to be answering questions from a group of people. I was told that it's good practice to say something like "that's a great question" every time someone asks anything, as a form of social lubrication, to encourage others to ask questions. I can't say whether it works, and it was advice for a spoken context rather than written, but I don't know how to finish this sentence.

In a spoken Q&A setting opening every single response with "that's a great question" or "thanks for asking that" or whatever is pretty common as a way to fill a few seconds while you think about your response. This is obviously unnecessary on slack.

Well before the LLM explosion I would often preface my answers with some form of praise for the question. It depends a lot on audience of course, but it’s amazing how many people tend to perceive direct answers to their questions as negative… and just as amazing how far a little strategic sycophancy goes to temper that. Even though everyone knows it’s half-sincere dead weight.

I would probably reply with "that is a great question" only as a euphemism for "I don't know"

I have only heard this phrase in american tv shows and movies

work at a crown corp and you'd change your stance on above...