Stealth by Karl Karlmann

Vanilla mobs notice players within a fixed radius. This mod gives them line of sight instead: walls block view, and darkness reduces how far a mob can see. A mob cannot spot a player through a solid wall two floors above it, and it no longer attacks from nowhere. Mobs can use the vanilla Night Vision effect to see better in the dark, but the mod leaves spawn logic untouched, so the effect has to be granted by the player or the server.

Hearing follows the acoustic system of the vanilla Warden. Mobs register footsteps, broken blocks and thrown items, and react to them by growing alert and searching the area around the source of the sound. The mod is published for Forge and NeoForge on 1.20.1 and 1.21.1 and runs on client and server, so both sides need it. The files are released under the Creative Commons Attribution Non Commercial 4.0 license, and the source code and issue tracker are public.