When trapdoors are placed at the top of a block, mobs can spawn on the following types:

• Warped trapdoor
• Crimson trapdoor
• Iron trapdoor

Mobs do not spawn on top the following top-placed trapdoors:

• Oak trapdoors
• Spruce trapdoors
• Jungle trapdoors
• Birch trapdoors
• Acacia trapdoors
• Dark oak trapdoors

Mobs did spawn on top of top-placed trapdoors in 1.14, 1.14.1 and 1.14.2, but it was changed in 1.14.3 I believe it was. Not sure if iron trap doors have been spawnable ever since, or if that was re-introduced in 1.16.

Personally, I think mobs should be able to spawn on top of them, when they are placed in the top of a block and are closed.
It's kind of the same as other non-full blocks placed in the top position, like upside-down stairs or a half slabs in the top position.

Either way, it should be consistent with all trapdoors.