Another explanation, at least for Germany, could be the "Netzwerkdurchsetzungsgesetz", a law that basically requires all social media to ban "obviously illegal" content in 24 hours or less or these platforms paying a high fine in the hundredthousands or even millions of € if I'm not wrong.

Because YouTube, Facebook, Twitter, etc. are very big it's not possible to check all content posted on them manually by employees. So all these platforms are either using an algorithm-based mechanism (f.e. YouTube, as is very well known and infamous) or a frequently abused user-based system via the report-function (f.e. Twitter) and therefore this law leads to overblocking. The german satire-magazine "Titanic" even was one of the first victims of this "Netzwerkdurchsetzungsgesetz".

So the reason behind a video-ban in Germany could at the end be just mentioning the word "holocaust" in the title as @davidpakman did and the YouTube-algorithm overreacting to it to make sure that YouTube doesn't need to pay a high fine but I'm not sure about possible reasons in other countries like UK.