I really enjoy using GNU Social and it is good to see alternative implementations using the same protocol. An option to just use Postgres, and not Postgres + Redis might make installing easier for experimenting with running a node.
Mastodon uses redis sorted sets and publish/subscribe for timelines and real-time message delivery respectively. And for background processing. It's not easily replaceable, plus it's a super lightweight dependency and a lot of projects nowadays use redis as well.