The current versions of FontAwesomeFX are only built for the latest JDK at the time they are released (currently Java 11) and are incompatible with older Java versions. The latest Java 8-compatible version (8.15) has some compatibility issues with Java 9+, so we are currently using an unofficial fork of FontAwesomeFX that provides compatibility with both Java 8 and 9+.
However, the unofficial fork hasn't been updated in a while, and the official developer does not seem to be interested in merging the patch. This means that as long as we want to keep supporting Java 8 (which will be a while) we won't be able to update FontAwesomeFX. This is not a great situation, so it might be a good idea to look for a better maintained alternative to FontAwesomeFX.
Also, the unofficial fork is hosted on a personal Maven repository on Bintray, not on a standard repository like Maven Central, JCenter or Sonatype. This is an issue for projects that depend on the ProB 2 UI (i. e. plugins), because (depending on the build setup) they then have to explicitly declare that personal repository, so that the transitive dependency on FontAwesomeFX can be resolved.