Hi - that is correct - we don’t allow users to add any nextcloud apps.
It turns out, managing a Nextcloud installation is quite complicated - and the apps is the primary source of that complication. For example, support we install the app Foo. Then, 2 users start using Foo. Then, a year goes by and we are ready to upgrade Nextcloud but wait - there’s a problem. The developers of the Foo app have abandoned it and it’s not compatible with the next version of Nextcloud. What do we do? We can upgrade Nextcloud and 2 users lose both Foo and the data in Foo. Or we delay the upgrade of Nextcloud and all users lose out.
To avoid this situation, we are very conservative about which Apps we add to our install - we only installs ones that we think a lot of users will use and that we are reasonable certain will be well maintained.
Mind Map seems well maintained, but I agree - I thnk it would be used by very few people so I don’t think it’s a good idea to add it.