I've seen nothing to suggest SSE would be (or is even capable of being) deprecated on HTTP/2.
You may be confusing it with HTTP/2 Push, where a server can inform a client of a future resource it will need, which is _de facto_ dead, with only a few real implementations beyond the bare minimum required by the HTTP/2 specification.
You may be confusing it with HTTP/2 Push, where a server can inform a client of a future resource it will need, which is _de facto_ dead, with only a few real implementations beyond the bare minimum required by the HTTP/2 specification.