You simply haven't viewed any conspiracy videos. My YouTube was very clean forever as well but a few days ago I saw a bright light in the sky (far brighter than any reasonable star or satellite I'd seen) and I googled around to see if there was anything about it online. Watched a YouTube video of a bright light in the sky with no explanation provided. Next day I watched a suggested Joe Rogan video taking about UFOs. Now a good third of my suggestions are UFO conspiracies. It's been getting better as I avoid them but YouTube was very quick to suggest multitudes of conspiracy because of a single Google search, and I had no history of interests like that.

So it's possible to have a reasonable and logical YouTube. However, even a single video can trigger YouTube's algorithm to attempt to pull you in.

I pay for Youtube Premium, but I find it curious (and objectionable) that their "View in Incognito" function (in Youtube) will not reflect Premium-ness. They show ads. They still know who you are -- because it's just an option to leave Incognito mode to get back to Premium/not-incognito mode.

Fwiw you can always reset dynamic recommendations with the "Clear History" button in the History tab.

Nice to do this every once in a while anyways, just to find another pocket of Youtube that interests you.