I have the old UFO shaped one with the micro USB cable, that thing continues to work for me with youtube but I dunno about anything older than that hardware revision.
I think it's a combination of things, one thing is youtube accidentally prevented playing ads on chromecast for the longest time, at least I assume it was accidental, and they've changed how things work to not make that happen anymore, though this change happened a bit further back (at least a year ago, I think a bit longer).
More recently though I think more 5-15 second ads which can't normally be skipped are being put into adsense, which has increased the number of ads that show up on chromecast, as the chromecast won't play ads which can be skipped from what I've noticed.