If you don’t mind, I will join this thread.
The mentioned link is useful in case of your own integration (you have your own storage and you integrated Document server (DS) with it). In case of using ready-to-go connector apps (https://www.onlyoffice.com/all-connectors.aspx) you can’t perform such actions, because you don’t have direct access to customization section. The connector apps are provided ‘as is’.
Nevertheless, probably there’s a workaround solution. The plugin manager is a plugin too. You can perform the same actions for it as for other plugins. Please check out this title: Adding new plugins to server editors - ONLYOFFICE
It describes manual plugin installation on the server.
the point is that you can remove the plugin manager folder and restart all services to disable this feature for all users.
Here’s my test where I removed this folder (and Macro folder too):
As you can see, plugin manager is no more available.
NB! I would recommend to prepare a copy of this folder just in case, if you decide to rollback changes.