Mig-21bis in VVS service

Does anybody know for how long did the Mig-21bis serve with the VVS? I have OOB from military balance in 1989-90 and World air power journal 1990 that still lists it as active

anybody have more details ? how long did it survive and given that the mig-29 was plentiful by late 80s ( almost 450 according to most sources in VVS) what possible reason did it serve ?