Well, to be fair, I did say "is not read beyond the code itself", header is not the code, so retry-after is a perfectly valid answer. I vaguely remember reading about it, but I don't recall seeing it used in practice. MDN link shows that Chrome derivatives support that header though, which makes it pretty darn widespread
Up until very recently I would have said definitely not, but we're talking about LLM scrapers, who knows how much they've got crammed into their context windows.
Almost certainly not by default, certainly not in any of the http libs I have used
If I find something useful there, I'll read it and code for it...