I guess we should have discussed the threshold for this. I naively thought just for new methods or classes. What do other libraries do?

Yeah I realized it was weird cause we use what's new for lots of what's changed and have almost no non-deprecation API changes

and have almost no non-deprecation API changes

Which is a good thing (API stability 👍). Additions/extensions are in "What's new". Almost all other changes should have deprecations.

Is there preference for the summary or blank version? If yes, should I document that in #24161? Also any styling preferences?

Is there preference for the summary or blank version? If yes, should I document that in #24161?

Whether a comment makes semse depends is case-by-case: If the whole thing (function/parameter/...) is new there is nothing more to say. If only part is new, like in the {}-style example, that should be documented. If you want to be very explicit, you can document that, but I would also be ok to not write that out in text but add one more example with such a summary.