SLF4J 1.7.23+ is required to enable `slf4j-log4j12` with MDC feature to run under Java 9. Also, this will bring all latest bug fixes.
When running under Java 9, log4j version 1.2.x is unable to correctly parse the "java.version" system property. Assuming an inccorect Java version, it proceeded to disable its MDC functionality. The slf4j-log4j12 module shipping in this release fixes the issue by tweaking MDC internals by reflection, allowing log4j to run under Java 9. See also SLF4J-393.