I have no idea when it stopped working, because I scarcely ever used the affected buttons until very recently—I have a vague feeling it might have worked three or four years ago, but in the last couple of months it’s definitely not been working for me.

uBlock Origin is the only extension that I have that could feasibly alter anything, and all it’s doing is blocking googleads.g.doubleclick.net. Nothing of interest shows up in the dev tools. 99.9% sure the bug is with YouTube, though in what particular way I cannot guess.