Create a boat spawner with 

/give @s spawner[block_entity_data={SpawnData:{entity:{id:"oak_boat"}},id:"mob_spawner",MaxNearbyEntities:1s}] 

and observe it spawn an oak boat. Then place down a spruce boat, destroy the oak boat, and see that it won't spawn another one because the MaxNearbyEntities count has been reached. This was previously an issue due to inheritance (see MC-259321), and was fixed by just comparing the entities class. This isn't sufficient anymore as now different entity types can have the exact same implementing class (all the boats except bamboo).