Did you mean the files_mindmap app?
The app itself is abandoned and is unfortunately no longer being developed by the maintainer. But the Nextcloud engineer artonge has made the most important adjustments to make it compatible with the vue-powered interface of Nextcloud greater 28.
I then built an installable version of it, which you can download → here ←
The limitations, that mindmap files cannot be opened when shared, will hopefully be solved with Nextcloud > 31, when the share UI will be ported to Vue as well.
hth.
Much and good luck,
ernolf