MySQL 8.0 Release Notes
        MySQL 8.0 Source Code Documentation
        
Adds a value to a comma-separated list of values and returns the result.
          This function and list_drop()
          can be useful for manipulating the value of system variables
          such as sql_mode and
          optimizer_switch that take a
          comma-separated list of values.
mysql> SELECT @@sql_mode;
+----------------------------------------+
| @@sql_mode                             |
+----------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES |
+----------------------------------------+
mysql> SET @@sql_mode = sys.list_add(@@sql_mode, 'NO_ENGINE_SUBSTITUTION');
mysql> SELECT @@sql_mode;
+---------------------------------------------------------------+
| @@sql_mode                                                    |
+---------------------------------------------------------------+
| ONLY_FULL_GROUP_BY,STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+---------------------------------------------------------------+
mysql> SET @@sql_mode = sys.list_drop(@@sql_mode, 'ONLY_FULL_GROUP_BY');
mysql> SELECT @@sql_mode;
+--------------------------------------------+
| @@sql_mode                                 |
+--------------------------------------------+
| STRICT_TRANS_TABLES,NO_ENGINE_SUBSTITUTION |
+--------------------------------------------+