9.0.10 Pixelmon, Technic Launcher, SMP

The external move "Ground Birds" does not appear to have an effect on Flying Pokémon.

This was tested with a Geodude with Smack Down (though reportedly the same was observed with an Arceus with Gravity by another player on the same server). When the external move is used underneath a flying wild Pokémon (e.g. a shiny Drifloon), the sound effect plays and the external move goes on cooldown, but the Pokémon continues flying without having been grounded.

Having experienced this myself, it does seem to have some effect, but not the instant grounding it did in previous versions. Nearby birds will no longer ascend, and they don't seem to take off again if they do land. I might be wrong, though, it can be kinda hard to gauge their actual height given the impossibility of hovering with the current flying system and the difficulty of just eyeballing it from the ground.