Backups of the JVM.CONFIG file are not created.
CF10 > Server Settings > Java and JVM in part says:
Backups of the jvm.config file are created when you hit the submit button. You can use this backup to restore from a critical change.
CF10 Help page indicates jvm.bak will be created:
Note Help indicates incorrectly JVM.CONFIG and BAK are in cf_root\runtime\bin where as JVM.CONFIG is located cf_root\instance\bin .
Thanks in advance, Carl.