to clarify, with "Temlate DB MySql works on mysql 8.4", I mean works to monitor that mysql version but lacking replication monitoring support

It's not just a template problem. I investigated the code of zabbix-agent2 and found that even in the latest versions only the “show slave status” command is used in the MySQL plugin code

The temporary fix for me it is build custom zabbix_agent2 with next fixes 

# Files to change
zabbix-src/src/go/plugins/mysql/
 handler_replication_status.go
 handler_replication_discovery.go

handler_replication_status.go (lines 26, 31)

.....
const masterKey = "Source_Host"
.....
         rows, err := conn.Query(ctx, `SHOW REPLICA STATUS`)
.....

handler_replication_discovery.go (lines 28, 39)

.....
         rows, err := conn.Query(ctx, `SHOW REPLICA STATUS`)
.....
                 res = append(res, map[string]string{"Source_Host": row["Source_Host"]})

Also the templates key values must be changed according to the new values from zabbix agent

Templates > MySQL Zabbix agent 2 > Discovery rules > Replication discovery > Item prototypes

Be careful.
These changes are not backward compatible and will not work with MySQL versions that do not have “show replica status” command

Thanks for the potential fix master2sid2 

I use zabbix_agent (v1) but it should be similar here.

This started to become an important issue after upgrading to mysql 8.4 LTS. The issue affects all zabbix versions including 7.0 LTS. 

I think, the pllugins must check the mysql version, run appropriate command and then replace "replica" and "source" keywords with "slave" and "master" to support existing templates. This way only the plugin code changes, and nobody has to apply any migration.