If by this you mean:
List<Foo> foo = new List<Foo>();
Gets converted to:
List<Foo> foo = new List<>();
This has always been supported in JDK7. It wasn’t in JDK6. Any code you see like that is fine to convert even if we don’t move to JDK8.
I feel like dropping JDK7 support is essentially cutting off a large swath of Android support. I don’t use it so I don’t “have a dog in that fight”… but I know others do and might miss being able to use JME for their projects. “But but… the latest phones will work just fine maybe with some tweaks and stuff because we haven’t tested it…” Even so, that cuts off a LOT of potential customers in a market that is already going to be small enough.
Personally, I don’t see a strong reason to move to JDK11 with any urgency. It’s really a shame that Java is getting messed up in totally incompatible ways. It had a long run, though. Given relative time scales, it could be that some monkey makes JME4 before it becomes an urgent problem.