I was not asking about Wasabi, I read up on them and have no interest in what they say can deliver.
It is not what they say they can, they have delivered! Wasabi is an amazing wallet, which provides decentralized permission-less mixing - think of it as the next level of your classic mixers. This is probably why the user, on their own, did this. They want to promote this amazing wallet - because actually amazing wallets do not have marketing budgets like all the multi-coin garbage like Coinomi or Enjin.
Only him? Nobody else?
It's the only user I saw with, but there might be more.
Afaik these signatures are not copy-righted and publicly available, so I guess everybody is free to use or modify them. I always thought, that this could lead to issues, not sure what this Wasabi one is about though.
No. You are not allowed to plagiarize anybody's signature design here.