I believe they’re officially deprecated. They were never in use much, with the Soviet Union collapsing long before the average ex-Soviet citizen had access to the internet. However, it was still reserved, and is still in use today. Russians and Russia affiliated countries seem to like the TLD.

I don’t think .su has any regional restrictions to the signup process. However, being a Russian TLD, getting one in the west may be risky and possibly even difficult because of sanctions.