The MSSQL plugin for Zabbix Agent 2 is documented in the Zabbix documentation.

It says that the plugin "is available and fully described in the MSSQL plugin repository."

Also, the Plugins documentation, as well as the Windows agent installation documentation, says that this is a loadable plugin and must be be installed separately, again, pointing to the git repository.

However, when searching the repository, there's no MSI or exe file available to download, nor a download link.
The official Zabbix download page does not provide a separate plugin download either.

Without any hint, we can only find the actual plugin by browsing the Zabbix CDN.

So, it seems to me that the Zabbix documentation and Zabbix's Git are missing the plugin download reference.
It would be better to have the plugin download available on the official Agent download page.