Multiple mods add mobs that dont despawn. So far I have found drowned that cant despawn and illager patrols that dont despawn. Unfortunately, over time this makes the world less performant and can cause problems. At the point that i quit the world I was up to 25 drowned that were persistent without holding any items and without any armor.

for early game, this makes things unplayable, especially if any of these mobs spawn as an unkillable eldritch mob since there is no other way to get rid of it without cheating (and arguably the pack should not require cheating)

for lat game, this makes things unplayable because the number of persistent entities increases to a point where the world chugs through entity AI and is no longer able to process them in a single tick...