(Note: This may just be a documentation issue, to describe the expected behavior...)
The problem here is visibility in that there seems to be events in the "background" that continue to process, but no way to see that a trigger is active (as the item reports as "Not Supported").
1. /tr_events.php?triggerid=ttt&eventid=eee (Event that has the documented "message actions" as it continues to escalate)
2. Clicking on the item from the ^ event page: /items.php?form=update&itemid=iii --> "Not Supported"
3. /events.php?groupid=ggg&hostid=hhh (*no triggers, ttt, visible on this page*)
4. /triggers.php?form=update&hostid=hhh&triggerid=ttt OR /triggers.php?hostid=hhh – reports: "timeout while executing a shell script"
It seems like the event should still be visible in the /events.php page even if the trigger is no longer available (b/c the item is no longer supported).