I don't recall there being an actual rule saying you could tag descriptions, I think that was more of a community rule (which I've learned to be very cautious of now days) like people would only report vids if the tags were irrelevant. I do recall last year a lot of people were saying that tags in the description were okay as long as it was relevant, but YouTube's ToS stood as it was, meta data and blocks of keywords in the description and the ToS prohibited it at the time regardless of the blocks of keywords being relevant or not.
"It is a violation of YouTube’s Terms of Service to use misleading metadata and blocks of keywords in your description field.". This is probably where a massive chunk of the confusion was caused in the community, people thought "hey my tags aren't misleading so I can pop them in the description."
This is partially why I did it when I started back then, so many people were saying it was okay to do as long as it was relevant and when I had checked to be sure; that's how I interpreted that part as well, by reading "...misleading meta data and blocks of keywords..." as misleading meta data or misleading blocks of keywords, to me that's what I understood it to mean but it doesn't. It wasn't until around Jan-Feb this year that I went through every single 1 of my old and even newish videos and removed any excess tags. I think YouTube are only now realising the problem is getting out of hand, changing retention to the biggest ranking factor was only the tip of the iceberg back then. What I think they should do is add quick permanent tutorials for any newbies and even old channels who still haven't read the ToS, just to combat this crap a little better and also avoid people crying over it, some still get their channels back with enough luck, some deserve to stay on YouTube, some don't though.