Thanks a lot for the reply. I asked the question here because I only joined portlet-spec as (former) EG member right now. And the question will affect both over time.
I don't see why JSR-378 should not go final unles the EC believes otherwise (at the moment I am not there) as it targets Java EE 7. Even a Portlet 3.1 or 4.0 spec for Java EE 8 in theory could be an option, but I guess especially IBM with a large stake in EE4J, MicroProfile or other projects like OpenJ9 may ask itself, if Java EE 8 was worth the update or skipping it and using EE4J or whatever the platform ends up named as instead.