That is if the connection isn't downgraded by some other mechanism. Lots of corporate clients downgrade connections to HTTP/1.1 due to how their network is setup.

There is also an issue with SSE I've noticed that websockets doesn't seem to have as much of a prevalent issue with, and that is being blocked by corporate firewalls.

Most websites use TLS, HSTS and the like nowadays. It’s hard for a corporate proxy to downgrade the connection.

Also, the limitation is only applied by browsers, not by (reverse) proxies and other non-browsers clients.

That being said, Mercure allow to subscribe to many "topics" using a single HTTP connection. This helps mitigating the limitation with HTTP/1 (which is very uncommon these days).