I wish there was a power type that prevented other aggressive mobs from targeting them.

아푸글리에는 이런 특징이 있다 apugli:mobs_ignore.

Apugli only supports up to 1.20.4... Is there any way to use it in 1.21.1?

Apugli has this feature as apugli:mobs_ignore.

Unfortunately, Apugli does not support 1.21.1 and does not intend to.

EDIT: I've actually decided to implement this power for the latest alpha myself, based on Apugli's existing code. You can find it here. The JAR you will need can be found here.

The Apugli dev is also reportedly planning to PR most of their custom power types/conditions/actions into Apoli itself, post-1.21.1