In Minecraft version 1.17.1, light levels follow the "Legacy" rules used for most of the game's history. It is important to note that the major overhaul where hostile mobs only spawn at light level 0 did occur until the 1.18 update.
In 1.17.1, hostile mobs (Zombies, Skeletons, Creepers, etc.) can spawn if the . Light Levels 1.17.1
: Glow Lichen (introduced in 1.17), which is often not bright enough on its own to stop mob spawns. 4. Technical Tools In Minecraft version 1
: Maximum brightness (direct sunlight or standing on a Glowstone block). Level 0 : Absolute darkness. In Minecraft version 1.17.1