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.