I don’t know much about it myself, but I would guess it is negligible. Maybe for small propeller machines with a fairly limited amount of fuel capacity; but larger planes, especially commercial ones, have reserve fuel for quite some time.

Situations where landing at the destination is temporarily unavailable, air traffic requires the plane to circle for some time, or they are even rerouted to a different airport can always occur and are accounted for. A minor increase from rainfall shouldn’t make a dent. I would think.

I just yesterday saw a post how disabling downvotes for an instance that still acts as part of the greater fediverse causes issues and unfair advantages with posts being created there that can achieve a high score very easily, even on accident.

I would expect Lemmy to rework that function to avoid these issues so that up- and downvotes are either mandatory or can only be disabled together. I would not recommend trying to find an instance based on if they practice this most likely not here to stay functionality.