The next scheduled major release of Openfire is version 4.2.0. The current thought is that once 4.2 is branched, we will march master branch to support only Java 8+. Community feedback is certainly welcome.
Note; until we upgrade to Jetty 9.3.6 or higher, it's not possible to use Java 8 idioms (e.g. lamdas) in JSP pages.
It's necessary to set the sourceVersion/targetVersion in the Jetty Jspc Maven Plugin, something not available before that version.
Maybe you should create new ticket for this, so it won't get overlooked and maybe also a ticket to update Jetty at some point and revert jspc changes.