Pythagoras building mod has iron bars to make jail cells. Sexiles and Slaver mods have the crucifix cross, meat hook, leash tie downs, cuffs, and truncheon/leash for non-npc’s. I think the iron bars should definitely be added to the base game (possibly as a t2?), but the slaving/torture stuff is fine in mods where private servers can prevent them from being abused on unmoderated servers.
The idea of expanded thrall interactions fits right in with an AI Overhaul for NPC's. Those things would really help fix the game in some aspects and bring more immersion to the players.