I wrote a PubSub implementation that uses Firebase Realtime Database as a message broker, I recently broke it out of the project where it's been in production for a while and polished it up a little, and it should do its job well enough. I'd be happy to have it included in the list of PubSub implementation if you deem it fits
"companion" PR to https://github.com/apollographql/apollo-server/pull/6059