I've tried using Karaf 4.3.0 built from sources (to fix another issue) and on Java 11 now am always getting following message in console:
It appears it comes from log4j (specifically this line) and is raised because on Java 11 reflection from Java8 does not work and "org.ops4j.pax.logging" does not include java9-compatible classes most probably because of this issue.
This message does not appear in Karaf 4.2.6 and after some debugging I see this is happening because in 4.2.6 version of "org.ops4j.pax.logging" is 1.10.2 and code goes through this line:
getServices() returns null and therefore event.getLocationInformation() is not invoked
In "org.ops4j.pax.logging" v1.11.0 (included in 4.3.0) code goes through this line:
because getServices(<empty array>) returns that same empty array, code falls into 'if' with that empty array and event.getLocationInformation(); is called (what throws the subject WARNING), whereas its output is not used because list of appenders is empty - no event.getLocationInformation() == no WARNING
So ... it seems it is bug in "org.ops4j.pax.logging" v1.11.0 and it is to early to include it into Karaf.