Shulkers prevent other hostile mobs from spawning, despite the fact that they neither spawn nor despawn naturally. Naming the shulker or placing them in a boat will cause them to stop counting towards the mob cap, which is intended per MC-182897, even though nothing changed. They can't despawn regardless, so they should never be counted towards the mob cap.

Almost all other mobs that don't despawn naturally will not be counted towards the mob cap: this includes:

• Mobs that have picked up loot
• Mobs in boats
• Elder Guardians
• Named mobs