Mod Farsighted Mobs (Forge) by Sindarin27

Forge version. See this page for the Fabric version.

During your many playthroughs of Minecraft, you may have noticed the extreme nearsightedness of mobs in the game. As long as you stay more than 16 blocks away from a creeper, it will never come to blow you up. Since mobs do not spawn within 24 blocks of the player, this means it is easy to survive: just stay still and there is only a minuscule chance a mob will wander close enough.

No more of that. By default, this mod increases the follow range of every hostile mob to at least 32 blocks, making the game a lot harder. This behaviour is fully configurable from the config. Not only can the minimum range of all hostile mobs be changed this way, you can even override individual mob types. Want all skeletons to be snipers that can spot you from a hundred blocks away? Do you prefer zombies to be so blind you have to literally bump into them? The power is yours!
 
This mod is entirely server-sided and is not required on the client side.

Note that only newly-spawned mobs are affected. Mobs that were already in the world before you installed the mod or changed the config will not be affected.