I recently built a new PC and, while I didn't migrate everything I saved, I made sure to keep my old system around for a few weeks just in case I needed anything.
Someone in a community I was in asked if someone had a link to some old Call of Duty shitpost from a decade ago, and I immediately recognised what they were on about. But a good hour spent scouring my early YouTube likes revealed nothing, so I dragged the old rig back in, powered it on, lo and behold within a minute I'd found the exact video I must've saved eons back.
No idea who created the said video, but I'm sure that if I noted a name or channel URL, there'd be a lot more missing media out there which others would be looking for.
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On one hand, I respect the fact that YouTube facilitates a level of privacy when you delete or hide things, obfuscating any trace of the video from playlists, etc. But on the other hand, the amount of times I've known I've added a video to a playlist only for it to be deleted with no trace... Yeah, it can suck. Recently turning a lot of unlisted videos into private ones hasn't helped in that respect either.