You just either make some QoL improvements with curses, make your golem a bit better, or invest more into Bone Armor if memory serves. I can’t speak for every class but yeah… functionally nothing really changes on Skelemancer/Zoomancer after, I think, 87.
I did it once on my HC Necro but that was just a function of playing more than a function of trying… I just REALLY liked the character and played it a lot for a couple of years so hit 99. I’m sure there is someone out there who cared but yeah… hitting 99 was never a real part of D2’s culture at all. the next day i’m back up and rolling again. Highest i’ve had was 97, i always end up dueling and get clapped at some point.