You can use `Maybe Void` to drop a field from a JSON object

maybe-void-aeson.hs

{-# LANGUAGE ImportQualifiedPost #-}
{-# LANGUAGE DeriveGeneric #-}

module MaybeVoidAeson where

import Data.Aeson qualified as Ae
import GHC.Generics

data T a = T { x :: Int, y :: a } deriving Generic

instance {-# OVERLAPPING #-}  Ae.ToJSON a => Ae.ToJSON (T (Maybe a)) where
  toJSON = Ae.genericToJSON Ae.defaultOptions { Ae.omitNothingFields = True }
instance Ae.ToJSON a => Ae.ToJSON (T a)

{-
λ Ae.encode $ T 2 (Nothing :: Maybe Void)
"{\"x\":2}"
-}