This limitation is gone with HTTP/2 and HTTP/3 and the Mercure hub automatically use the most recent protocol supported by the browser.
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.
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).