MonsterEyes

Very simple fabric mod, monsters now only attack when you are in line of sight

It also supports the backstab effect, which deals additional damage when attacking outside of the monster's field of view.

This mods uses the Cloth Config API, and through Config, you can set the field of view for each monster or adjust the backstab damage multiplier.

It works just by putting it on the server side. In that case, reload the config with the /monstereyes reload command.

Based on version 1.1 or earlier

The entity has an attribute monstereyes:field_of_view. The default value is 180, and the viewing angle determines the range at which you are recognized.

There is also a backstep function that deals additional damage when attacking a monster outside of the field of view, and this can be adjusted with the game rules doBackstabDamage and backstabDamageMultiplier.