The endorsed standards override mechanism and the extensions mechanism were removed in JDK 9 via JEP 220.

As part of this, code was added to Arguments::finalize_vm_init_args to check for the endorsed and ext directories so that the VM terminates if they exist. These checks are no longer needed in JDK 10 and can be removed.

The check to be removed is here:
http://hg.openjdk.java.net/jdk/hs/file/eebf559c9e0d/src/hotspot/share/runtime/arguments.cpp#l3385

  if (CheckEndorsedAndExtDirs) {
    int nonEmptyDirs = 0;
    // check endorsed directory
    nonEmptyDirs += check_non_empty_dirs(path);
    // check the extension directories
    nonEmptyDirs += check_non_empty_dirs(Arguments::get_ext_dirs());
    if (nonEmptyDirs > 0) {
      return JNI_ERR;
    }
  }