If a field is not set in the raider entity sub-predicate, the game will check if that field is false. For example, if is_captain is not set, only non-captains will be matched.

To reproduce, enable the attached datapack containing the tis:not_in_raid predicate.

{
  "condition": "minecraft:entity_properties",
  "entity": "this",
  "predicate": {
    "type": "#minecraft:raiders",
    "type_specific": {
      "type": "minecraft:raider",
      "has_raid": false
    }
  }
} 

Then go to a pillager outpost, find a captain and non-captain pillager, and run the following command:

/effect give @e[distance=..24,predicate=tis:not_in_raid] minecraft:glowing 10 0 true 

The expected behavior is that both the captain and non-captain will glow, but only the non-captain glows.