By default, Lemmy allows downvotes globally. However, when a server disables downvoting, it is similar to using a feature that is usually reserved for enterprises and very small, non-federated communities.
If a user prefer to not see downvotes, they can disable it by his favourite client settings, but the rest of the community should not miss this functionality for the pleasure of few users.
they’re disabled on hexbear.net, take a wild guess why
spoiler
it’s to protect their circlejerk
Originated and enabled well before there were other active Lemmy instances or possibly even federation of any sort. It was put in place to stop anti-trans harassment.
Plus AFAIK it’s only truly applicable within Hexbear itself. The main effect it has is that a Hexbear user can’t downvote anything on any instance with their account.