The latest Byteman will run on JDK11 but it is built using a version of ASM that does not accept bytecode files as compiled by JDK11. The real problem is not specifically the bytecode contents but rather the version number. This can be fixed by moving up to the latest version of ASM.
JDK11 also allows for nestmate access (see the Java language spec for full details, but essentially it involves a loosening up of access rules for non-public members). Byteman probably ought to take account of these looser permissions when running on JDK11 but this is not really important since it already enables this sort of access anyway. That detail will be addressed by a related issue.