Clay Golems have, I think, always had that ability. It's nasty, and that 5e has kept it is IMO a good thing.
So if the party gets into a fight with a Clay Golem and some characters take damage, maybe they'll just have to live with a lower h.p. max from here on. While they'll logically want to get it fixed, nothing anywhere says they MUST be able to, or be able to easily.![]()
The oldest form of clay golem (The Strategic Review Vol 1. No 4.) had nothing special about its damage. When it was presented in the AD&D Monster Manual, its damage required healing from a 17th level or higher caster! (Not a restoration spell, just healing).
I recently revisited it in 5E terms: Dungeons & Dragons Lore: Old-School Golems - Merric's Musings
It's the resistances that make old golems a massive challenge. (I'm not a fan of how 3E added lots of ways to get around their resistances).
Cheers!