Server.properties spawn-animals/spawn-npcs affects summoning, but spawn-monsters does not

If you set spawn-animals=false in the server.properties,
then you can't summon an animal (the mob instantly disappears).

If you set spawn-npcs=false, you can't summon villagers either.

But if you instead set spawn-monsters=false in the server.properties,
that doesn't stop you from summoning anything.