What steps will reproduce the problem?
1. Send stanzas to many remote domains simultaneously.
What is the expected output?
Prosody should limit the number of pending s2s connections to avoid excessive resource consumption. Probably the oldest connection should be closed with an error if it is older than a configured time threshold. Otherwise new connections should be rejected with resource-constraint.
What do you see instead?
Prosody allows an unlimited of pending s2s connections, which may stay open for some time if the connection times out.