Sure. Absinthe.Subscription — absinthe v1.11.0 has some notes but they’re pretty outdated and the language speaks of a “beta” release which is obviously very old.
Again it depends a lot on whether those subscribers are all subscribed to the same thing or not. You can easily have tens of thousands of subscribers if they’re spread out over thousands of topics. When you get concentrations of subscribers though then this can definitely be a challenge.