Non functioning seat belts was the most common, on the occasions when there were cabin crew (sometimes there were none) they just told you to tie the belt in a knot if the buckle wasn't there or didn't work
My trips to the FSU did not start until 2001 so cannot comment before. Have seen some pretty tired seat belts, but only once have I had to tie the thing because the buckle was not working (an Aeroflot Don TU154). Felt like clapping when we landed with that one.