We didn't track version changes before 4.0 very well. There are many other objects added before 4.0 that also don't have a versionadded note, so I'd rather not add it for just one of them.

4.0 is almost four years old at this point so I don't see much point in tracking down exactly when in the 3.* series each object was added.

I see. The reason I opened this is that I only needed this single feature and wanted to define my dependency on typing-extension in the loosest (yet correct) way, so I went to search this specific bit of information and added it back when I got it. I understand if only doing this (very) partially isn't desired. However, I would suggest adding a note somewhere saying that whenever this information is missing, 4.0.0 is a safe minimum requirement to assume, because this isn't obvious at all from the outside.