When I use this rule,they are no longer spawn anywhere.....
And "dimension": "mmorpg:resettable_dungeon" is the same situation.
(I just want to know how it works.)

{
"mob": ["mutantbeasts:mutant_creeper","mutantbeasts:mutant_zombie","mutantbeasts:mutant_skeleton"],
"dimensionmod": "mineandslash",
"onjoin": true,
"result": "deny"
}

Well this rule would make those mobs not spawn in all dimensions from mineandslash. Is that what you want?

Yes,but it works on other dimensions..

OK,thanks

Ah you use dimensionmod. That's not supported in 1.15.2. In fact 1.15.2 is ancient. That's not a version I support still