Yes, I think Beehaw for example has no downvotes (or doesn't show them).
They're the safe space instance so it makes sense why. :B
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.
Beehaw and my instance are the only ones that I know of
This page, "Awesome Lemmy instances", has a big list of instances with some info, including whether they allow downvotes. I'm not sure if it's up to date though.
