Kinda not related but there was a thread about xyz domains: https://news.ycombinator.com/item?id=28554400

Also a lot that are just

"Yes <link to another youtube video> xyz"

I don't get the point.

Training the classifier that "xyz" is a sign of quality (by posting a benign, non-spam link) so that they can then use it for spam and have it go through?

That would be next level spam. But it sounds entirely possible.