Can javac 11 compile binaries/jars that will run on a java 8 JVM?
If so, with what flags?
Will setting gradle source compatibility to 11 and target compatibility to 8 work?
Yes, but not with source=11 and target=8; only source=8 target=8 will work. You can't use any java features introduced in java9, 10, or 11.
On the command line, the javac option you're looking for is -release 8
(which is like -source 8 -target 8
but shorter and better).