EntityType.FIREWORK_ROCKET has the `independent` field marked as false, which by javadocs indicates that it should not be spawned (or spawnable) via World#spawn(..), but is definitely spawnable via that method. I'm not sure if this is a relic from older versions that needed a special method, but I don't think it's correct now.

Since a firework rocket can be spawned via World#spawn (and is the only way to spawn one without a ProjectileSource?), I think this field should be set to true going forward, so EntityType#isSpawnable() returns the proper result.

https://hub.spigotmc.org/stash/projects/SPIGOT/repos/bukkit/browse/src/main/java/org/bukkit/entity/EntityType.java#122