System variable session_track_user_variables isn't documented

MariaDB [test]> select * from information_schema.system_variables where variable_name = 'session_track_user_variables' \G

*************************** 1. row ***************************

        VARIABLE_NAME: SESSION_TRACK_USER_VARIABLES

        SESSION_VALUE: OFF

         GLOBAL_VALUE: OFF

  GLOBAL_VALUE_ORIGIN: COMPILE-TIME

        DEFAULT_VALUE: OFF

       VARIABLE_SCOPE: SESSION

        VARIABLE_TYPE: BOOLEAN

     VARIABLE_COMMENT: Track changes to user variables.

    NUMERIC_MIN_VALUE: NULL

    NUMERIC_MAX_VALUE: NULL

   NUMERIC_BLOCK_SIZE: NULL

      ENUM_VALUE_LIST: OFF,ON

            READ_ONLY: NO

COMMAND_LINE_ARGUMENT: OPTIONAL

    GLOBAL_VALUE_PATH: NULL

1 row in set (0.006 sec)