As they are exclusively found as part of the fight with King Slime, they are similarly unaffected by the immunity granted by the Royal Gel and will continue to fire projectiles and damage the player on contact.
Although it is normally impossible to encounter a Spiked Slime on a normal world (without external modifications), it will still have 50 HP instead of 100 if one manages to spawn it inside a normal world.
It is not required for the Gelatin World Tour achievement.
While other enemies spawned by bosses, such as the Servants of Cthulhu, have a Bestiary entry, the Spiked Slime does not. This is likely because it is an Expert Mode-exclusive enemy and if it did have a Bestiary entry, it would be impossible to complete it fully in a Classic world.