System tables cannot be versioned (confirm limitation and document)

MariaDB [mysql]> alter table mysql.event add system versioning;

Query OK, 0 rows affected (0.04 sec)

Records: 0  Duplicates: 0  Warnings: 0

MariaDB [mysql]> create event ev on schedule every 1 hour do set @a=1;

ERROR 1545 (HY000): Failed to open mysql.event

MariaDB [mysql]> flush tables;

Query OK, 0 rows affected (0.02 sec)

# In error log:

2018-01-14  2:30:01 9 [ERROR] Column count of mysql.event is wrong. Expected 22, found 24. The table is probably corrupted

MariaDB [mysql]> alter table mysql.user add system versioning;

Query OK, 6 rows affected (0.04 sec)

Records: 6  Duplicates: 0  Warnings: 0

MariaDB [mysql]> create user foo@localhost;

Query OK, 0 rows affected (0.01 sec)

MariaDB [mysql]> select user, host from mysql.user where user = 'foo';

Empty set (0.00 sec)

MariaDB [mysql]> select user, host, row_start, row_end from mysql.user for system_time all where user = 'foo';

+------+-----------+-----------+----------------------------+

| user | host      | row_start | row_end                    |

+------+-----------+-----------+----------------------------+

| foo  | localhost | NULL      | 0000-00-00 00:00:00.000000 |

+------+-----------+-----------+----------------------------+

1 row in set (0.01 sec)