I maintain the Mercure protocol (built on SSE) and the reference implementation (written in Go, available as a standalone binary and a Caddy module) which does exactly that: https://mercure.rocks
In addition to the free and open source server, we also provide a cloud offering and on-premises versions that support clustering using Redis Streams, Kafka, Pulsar or Postgres LISTEN/NOTIFY as backends.
The solution is used by many big actors in production for years: