In 6.0 it's only possible to add a single LDAP server:

https://www.zabbix.com/documentation/6.0/en/manual/web_interface/frontend_sections/administration/authentication#ldap-authentication

https://www.zabbix.com/documentation/6.0/en/manual/api/reference/authentication/object

After 6.0, e.g. 6.4, you are able to add multiple server:

https://www.zabbix.com/documentation/6.4/en/manual/web_interface/frontend_sections/users/authentication/ldap#multiple-servers

These servers are however not possible to manage through the API:

https://www.zabbix.com/documentation/6.4/en/manual/api/reference/authentication/object

So: After 6.0 it is not possible to manage LDAP authentication servers through the API.

PS. I filed this as an issue rather than a feature request. It could be considered both, I guess.