I think it’s fair to say that the Sith Pureblood’s language, though obviously quite different before the colonisation of their homeworld and their culture by the exiled Jedi, is still around and still spoken.
I personally think that Pureblood families will speak it at home and have (Imperial) Basic as almost a second language.
For Sith practioners and those from long standing Sith families the language is spoken in a more formal or official capacity (the latter group are more likely to be fluent or more than passingly familiar with the language). Alien or non-Imperial Sith (ex-Jedi, older initiates coming from colonies, etc.) will be expected to learn the language but not necessarily assisted. The language and by extension the culture of the Sith - both in its religious-political sense and the pre-exiting Pureblood culture that draws from - are kept away from them as another way othering and separating the “lessers” from the “true” Sith.
Imperial Basic has a lot of loan words and concepts from the Sith language as a result but fluency outside of Pureblood or Sith lineage families is rare. It’s not entirely unusual for a Force-blind child in a Sith lineage household to not know the language.
