For Soviet-nostalgia buffs looking for a domain, although most obvious choices like CCCP, KGB, Lenin, Stalin and even Gorbachev and perestroika are registered under .su, GLASNOST.SU is available.
AFAIK IDNs are still possible under .su, but may depend on the registrar (non-Russian resellers may not do it).
If you mean what Russian words are available under .su, probably most, for example Ленин (Lenin) and Кремль (Kremlin), though КГБ (KGB), СССР (USSR), Россия (Russia) are registered, as are single letters like А or Х that people may like for a 'short' url.
Although the USSR collapsed not long after the creation of .su, it was briefly in 'proper' use on the internet - unlike, say, .dd, intended for East Germany - most notably via the ISP Demos, which famously used the name kremvax.demos.su as the name of its Usenet site, in reference to an early internet hoax/April Fool's from 1984 (https://godfatherof.nl/kremvax.html).
For Soviet-nostalgia buffs looking for a domain, although most obvious choices like CCCP, KGB, Lenin, Stalin and even Gorbachev and perestroika are registered under .su, GLASNOST.SU is available.