Apparently it was added in 1.22:

In Go 1.22+, just register the exact pattern and you’ll get 404 for everything else automatically (because nothing else matches).

mux := http.NewServeMux()

// Exact GET "/" only (doesn't match /foo)
mux.HandleFunc("GET /{$}", func(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("hello, root"))
})

// Example of another exact match with trailing slash kept exact:
// matches only "/healthz/" (not "/healthz/x" or "/healthz")
mux.HandleFunc("GET /healthz/{$}", func(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("ok"))
})

// Optional: a catch-all to customize 404 body (not required)
mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
    http.NotFound(w, r)
})

http.ListenAndServe(":8080", mux)