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It's certainly not very friendly if you want to remove categories en masse from your recommendations, but you can remove videos from your YouTube history list. There's even a search bar. I have found spending the time to purge a certain type of video from that history does improve my recommendations. Additionally, on the home page, you can click on the options overflow on each video and tell YouTube you don't want to watch it, and why.

You can remove individual videos from your watch history and this does appear to affect your feed shortly after.

On mobile this is under Library > History.

You can already do it on youtube, it's integrated on google activity.

https://myactivity.google.com/myactivity