Thanks for all the responses! It may help to distinguish two types of speculation (cf. https://terrytao.wordpress.com/career-advice/theres-more-to-mathematics-than-rigour-and-proofs/): "pre-rigorous speculation", in which one asks "dumb" questions (cf. https://terrytao.wordpress.com/career-advice/ask-yourself-dumb-questions-and-answer-them/) before fully knowing the field, and "post-rigorous speculation", in which one shares informed insights and opinions from one's rigorous understanding of the field.
I think we should encourage both types, in appropriate venues of course (and separated from traditional "rigorous" work).
@tao - One "safe space" for airing less precisely formulated speculations seems to be blog articles. But blogging seems to have declined in popularity with young mathematicians.
Another is Twitter, though I've mainly seen category theorists speculating there, not so much other kinds of mathematicians.
I hope Mathstodon will become another place for informed speculation!