I think Kev has got it about right...
2013 is when they started focusing on making less higher quality (and price) basses, started using wenge then and using solid brass hardware.
They went a bit mainstream with the whole nu metal thing around the millennium and the quality and neck size reportedly got worse.
Pre 1997 is given as when the quality dropped according to forums... no idea why but mid 1992 they moved the factory to the former east Germany and fell out with Schaller which meant not using solid brass bits... so that seems to be a major change...
1991 or earlier are nice, often with thinner necks. My main bass is a 1991 streamer stage one which is amazing, although I’ve got Barts and an acg preamp in it. I’ve also a 1985 JD thumb bass which is amazing but I’m not jelling with as much but it’s pretty nice to have one of the first few thumb basses ever made