I think for Virtualmin there should not be any grading (Grade A,B,C…).
There should just be the supported list of OS (currently Grade A) and that is it. If Virtualmin does not work on an OS it should just not be on the list.
This would also encourage a user to pick a supported OS.
Enforce this in the installer. 
I cannot speak to Webmin.
Just a thought for the new year
or at least a big red warning “at your own risk” and perhaps a list of known suppliers of VM that do not pre mess with images could be expanded
I think the grading achieves this already.
Grade A needs to be robust.
I applaud the unstable tag so you can work with some more recent rhel derivatives like fedora for example. It is just an option.
That makes sense from a user perspective, and I generally agree that a clear “supported OS only” list reduces confusion.
If I remember correctly, in the past the Virtualmin installer was indeed more restrictive and would refuse installation on anything below Grade A (Debian 10 release?). That approach certainly guided users toward stable setups.
However, there is also the testing aspect: if installation is fully blocked, how are early adopters or contributors supposed to validate upcoming OS versions? Should that require explicitly using the testing / development branch (e.g. Virtualmin 8), or perhaps a clearly documented override flag for those who intentionally want to test unsupported systems?
This way, regular users are protected, while developers and testers can still provide early feedback that ultimately improves future “Grade A” support.
This is easy to fix. If Debian is a Grade A supported OS, then you can assume that the next version of Debian will be supported but to install on them you either use a --dev-version or that version of virtualmin allow you to install on the newer version of the OS for testing.
TL;DR I would not block any Debian as this is accepted as a Grade A OS, and you need to try newer versions for testing, but I would not allow installation on ZorinOS etc..
So it is more a flavour, than verion number restrictions although this is still required
Maybe this statement is better:
Virtualmin works on these OS…
It might Install on any other flavour of Linux but there is no support.
We already say in Docs → OS Support page:
We strongly recommend the current stable release of one of these systems for most users. We never support beta or pre-release versions of any OS.
And, later for Grade B OS we say:
The following operating systems are not recommended for new or intermediate users.
We also don’t make it easy to install on Grade B OS and communicate clearly during installation if this option is selected:
You are about to install Virtualmin 8 on a Grade B operating
system. Be advised that this OS version is not recommended for servers,
and may have bugs that could affect the performance and stability of
the system.
Certain features may not work as intended or might be unavailable on
this OS.
There’s nothing to do here! We shouldn’t remove Grade B systems support. Some people rely on it, and we should respect that.
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