The specification appears to be very similar to FeedAPI which I have been involved with:

https://github.com/vippsas/feedapi-spec

Our focus was kind of the opposite though: Remove the event broker instead of adding one.

If I understand correctly, Mercure allows you to push events to the broker and clients pull log-based events.

With FeedAPI we focused instead of removing the broker (RabbitMQ/Kafka/...) and let clients pull log-based events directly from backends.

Very similar protocol though.