I regret not archiving all the videos from the youtuber Myles Power (pseudoscience-debunker). He was one of my more favorite YouTubers but a few years ago he nuked his entire channel and all the videos on it (I suspect because his employer didn't like him arguing with people on the internet).

I wanted to rewatch some of his videos recently and as far as I can tell most of his videos are now lost media. I should have downloaded them when I had the chance.