Each media platform has its own guidelines, but seem to watch what the others are doing so it is possible YouTube would have restricted or close down his activity soon anyway.
Tom Watson is not currently in a position to compel YouTube to do anything in UK so I think he might have fed into Tommy Robinson's myth, but it is a close one for me.
I haven't read his letter as it is on Twitter and tends to lock up my iPad for short periods, but I saw it was on Parliamentary paper. I would be vaguely interested to know if he was acting on behalf of one or more constituents, or whether it was a personal project of his own, but either way I doubt it will make an impact on its own.
Which might be why he has published it - to show he is willing to make a stand. With the machinations within his party, I take a cynical view.