As of now the check_mysql supports a "slave check" (-S, --check-slave). This seems to utilize the SHOW SLAVE STATUS command.

Since MySQL 8.0 the "show slave status" command is deprecated and an alias for new "show replica status". Since MySQL 8.4 the "show slave status" has been removed, see Server and Status Variables and Options Added, Deprecated, or Removed in MySQL 8.4 since 8.0.

Now the plugin returns with 8.4 the following error:

slave query error: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'slave status' at line 1

Most likely referenced here: check_mysql.c#L215