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When this happens to me I just delete it from my watch history and my suggestions are immediately amended.

On the YouTube video suggestion page, clicking on the 3 dot (⋮) menu next to the title of a video (only visible by hovering), there is a "not interested" option. I don't know how effective it is, but it is there.

It is likely best to watch such questionable videos on an incognito tab of a browser with an empty cache.

Browsing on your logged-in google account is what caused this persistence.

Which works until it decides that the video is age restricted and requires you to login to view it.

One must assume YT has a blacklist of search terms/topics that won't persist in recommendations(?) - at my place of work these types of blacklists are only possible to add to "when people ask" (unfortunately) or if they're basic/obvious (NSFW terms or "words you can't say on the radio"). The phrase "roadkill" specifically came up today in a somewhat similar context.