.su is available for registration, I'm not sure what the "in a limited way" is about. In Russia it's used to communicate old-schoolness, approximately.
It definitely is. In Germany, somebody was selling fraudulent public transit e-tickets on an .su domain for a while last year.
Not sure who the “.su” was supposed to appeal to, but they were slightly cheaper than officially licensed ones, which probably helped more than the TLD :)
Outside Russia it’s limited to renewals of existing domains due to sanctions. I believe ICANN is trying to eliminate the domain entirely due to it being obsolete, although I don’t know if a formal timeframe has been established for that yet.
Not sure who the “.su” was supposed to appeal to, but they were slightly cheaper than officially licensed ones, which probably helped more than the TLD :)