I have installed ISPConfig 3.2.11 and i noticed that the ispconfig plugins are not present in the ispconfig installation folder. Why aren't they there? Are they no longer necessary?
I found instructions on installing the roundcube plugins: https://www.howtoforge.com/install-ispconfig-3-roundcube-plugins-on-debian-10/
These plugins are not part of ISPConfig and are not developed by the ISPConfig developers, so they have never been a part of ISPConfig. Also the plugins are not needed to use RoundCube with ISPConfig as ISPConfig provides an email user login, but you can use them of course.
Ok, i can activate the others plugin like "password" and "managesieve" changing the config file in /var/lib/roundcube/public_html/ , right?
No, these do not know how to connect to the ISPConfig API. If you want to use such functions from within RoundCube, use the ispconfig plugin.