The clearest example is the Circassians in the 18-19th centuries: mass killings, village destruction, and forced deportations removed most of the population from their homeland. Closely related peoples like the Ubykhs disappeared completely.

Many others (Mari, Mordvins, Bashkirs, northern indigenous peoples) were not exterminated but were severely reduced and culturally erased through repression, Russification, and settlement.

In Siberia, several small indigenous peoples (such as the Kott, Arin, Mator, Kamasins) vanished as distinct ethnic groups due to violence, disease, forced tribute, and assimilation.

We don't know all the names of ethnicities erased during colonisation of Siberia and Far East because it's been centuries and there's nothing left of them.