I think Game Freak just removed Levitate for consistency with its new description, not to "nerf" Gengar.
https://projectpokemon.org/research/sunmoon/gametext.txt
>By floating in the air, the Pokémon receives full immunity to all Ground-type moves.
Before Gen 7, the description was just:
>Gives full immunity to all Ground-type moves.
Gengar is clearly standing on the ground in its model, so they removed Levitate from it for flavor reasons.
>>29797510
Just use Pokémon Injection and give it Levitate again.
>>29797510
Since when does GF even give a shit about being consistent with descriptions
about 70% of all Pokedex descriptions are bullshit
>>29797510
>Gengar lost Levitate due to Sky Battles
Sky Trainers did literally everything wrong.
>>29797571
Fucking sky trainer hunks.
>>29797563
This