Reddit does offer the option to "use old Reddit" if you're signed in. I only learned about this recently, maybe it's common knowledge.
I often browse Reddit with Safari and there haven't been very many extensions available for it. Certainly none to do something simple like this, since it would have to be made available through the app store.
PS: Apple has announced that FireFox and Chrome extensions can be made available for Safari in Big Sur, although it seems like they have to go through a convoluted process and probably pay $99/year for the privilege.