My Pi4B supervised installation became unsupported recently. The background is here.
I have subsequently installed a Debian 11 system on a Pi4B that boots with Grub (based on this guide). I then did a supervised installation of HA and attended to these instructions. The result reports true for healthy and supported.
The leading to the unsupported message was updated.
All you need to to is run “sudo update-grub”. It is mentioned in https://github.com/home-assistant/supervisor/issues/3715#issuecomment-1176312079.
Here is the link to the solution:
Updated to latest supervisor and despite mentioned changes still get "unsupported installation" - #6 by CentralCommand.
I hope your setup will run smooth after the update-grub command ran.
Not for an RPi with a default installation of Debian 11 because it doesn’t use grub. That’s why jaalperin linked to specific instructions for installing Debian 11 that uses grub.
I should add that the time to do this is not trivial, and I would have installed HAOS instead if any of the distributed images had been willing to boot on my pi4B. What I didn’t do, and clearly should have, was to boot Pi OS to make sure the bootloader was up-to-date on my 4B. Oh well.
I’m willing to post an image of a generic system to save folks the hours it takes to do this from scratch. Any suggestions where to post a 4G file and how to get downloaders to hold me harmless?
One last, late question. Why do we care if the system is supported? What is the “support” that I’m chasing? My unsupported pi4B systems worked fine for over a year.
You have a good point and wanted to say I appreciate all you did and also shared with the folks like me in the same boat.
Most of us non-OS users don’t care about the installation being supported.
We choose to run the install method that allows us the freedom to do other stuff on the device apart from just being locked in to the HA ecosystem.
I think the only support you might be missing is the ability to get direct support for you from a dev on Discord or on github via a bug report. But if others are getting similar bugs then it will likely get attention and get fixed any way whether you have a supported install or not.
The only down side that I know of is when breaking changes like the above happen on a Supervised install. Then it’s unfortunately up to you to figure out what needs done to fix things. But the forum is always still here and we don’t care (well most of us…) if you have a supported install or not. 
My solution was to just get rid of my test Supervised install and be done with it since I don’t use it for production anyway. And now with all of the issues people are having with their HA on Supervised installs I’m glad I don’t. 
One more gratuitous comment. Pi4B computers, rather than being inexpensive, are expensive and unobtainable. I can’t get a 4B for less than $150, if I can get one at all. In the mean time, I can buy a used Lenovo M93p SFF computer for $50 to $70. Add a $22 PNY SSD and you get a much better supervised/supported HASS machine. The one place I’ve come to prefer a pi4B is as a router running OpenWRT. Guess I got off topic.
Yes. It’s understood that jaalpin uses a non standard setup with grub.
And as jaalpin mentioned the special setup with grub I replied to it with running “sudo update-grub” to solve the problem.
Can you explain what “all of the issues” mean to you?
Recently I had only one issue with my supervised docker installation as the related link to the solution to solve the unsupported condition did not mentioned the obvious use of the command “sudo update-grub” after editing the grub configuration. And even this was added to the document describing the solution very fast.
All of the reports in the last few days with people frantically trying to recover their dead systems after an auto update killed everything in the middle of the night.
This is not the first time something like this has happened. Maybe not a frequent thing but frequent enough that I’m glad I don’t use OS or Supervised.
I update my docker install at my convenience.