Using the underscore '_' as identifier was forbidden with JDK 9, however still allowed for JDK8. Hence the attached example coding should compile with the javac option '--release 8'.

This fails to work with JDK14 (and newer):
--------------
OpenJDK\14\bin\javac --release 8 UnderscoreEnum.java
UnderscoreEnum.java:4: error: enum constant expected here
        NULL(null), _("&&"), __("||"), OR("OR");
                    ^
UnderscoreEnum.java:4: warning: as of release 9, '_' is a keyword, and may not be used as an identifier
        NULL(null), _("&&"), __("||"), OR("OR");
                    ^
UnderscoreEnum.java:4: error: illegal start of type
        NULL(null), _("&&"), __("||"), OR("OR");
                      ^
UnderscoreEnum.java:9: error: illegal start of expression
        CCEnum(final String value) {
               ^
UnderscoreEnum.java:9: error: ';' expected
        CCEnum(final String value) {
                                 ^
4 errors
1 warning

With JDK13 or below, it still works nicely:
--------------
OpenJDK\13\bin\javac.exe --release 8 UnderscoreEnum.java
UnderscoreEnum.java:4: warning: as of release 9, '_' is a keyword, and may not be used as an identifier
        NULL(null), _("&&"), __("||"), OR("OR");
                    ^
1 warning

Simple identifiers named '_' work with --release 8 (or -source 8); but when used in enum the compatibility breaks.