Yeah that's a tough feature for something that's not feature cluttered like Jira. I'd recommend going with a standard bargain product like Scrumfast.com, creating a "small customer" project, and simply using epics to link particular small customers.
I've started to collect some alternatives here, as this is a re-occuring issue at F-Droid (which does not permit Firebase or other proprietary components).
Gotify is an open-source push notification server (you can explore its Swagger API doc)
Server Sent Events (SSE) are used by some apps like Tutanota
XMPP is used by several apps (of course including XMPP ...