I had some spare time to have a look on some parts of I-MSCP. I tried to update the Roundcube webmail, added PHP8.0 to I-MSCP and removed support for Debian Jessie:
I could successfully install I-MSCP on Debian Buster with the latest maintenance branch on Github.
I am not sure, if I chose the correct way, as I-MSCP is a really huge project with many sub repositories (and branches), it is hard to find the right starting point.
Great - thank you! Do you have experience with the upgrade process as well?