Ender dragons and withers does not have same spawning rules as regular hostile mobs, and they does not despawn, but they can still affect the mob cap.

Steps to reproduce

1. Create a void superflat world
2. Press F3
3. Summon the ender dragon and wither
4. Observe the mob cap (M:*)

→ The hostile mob cap is increased.

Observed result:

The ender dragons and withers does not affect the mob cap.

Expected result

The ender dragons and withers can affect the mob cap.