We are planning on keeping both jetty-9 and released later this year jetty-10 to be able to run on java 8, 9, 10 & 11
We are already coding support for java >= 9 (eg multi version jars) into the code, but are doing so with java 8 compatible code.
However, we have only started recently to test against java10 and are being hampered by other tools (eg maven) not yet working entirely with it.
As Jetty-10 has been delay and then delayed again... we might eventually revisit the decision and make it a java 9 only or maybe even a java 11 only release... if we see a need/reason. Currently there is no big push for jetty-10 as we have seen no demand for servlet 4.0, so we are keeping it open as a possible big feature release, but currently almost all of what we want to do can be done in jetty 9.4.x