Get rid of all backports that are installed?

Backport kernel are always installed, you can only influence that by lowering the priority via the environment variable OMV_APT_KERNEL_BACKPORTS_PINPRIORITY. See https://github.com/openmediava…-kernel-backports_pref.j2

Quote from prtigger1

omv-changebackports no

That is an omv-extras command that sets OMV_APT_USE_KERNEL_BACKPORTS to false, removes the backports repo, and removes the backports pinning.

Quote from prtigger1

Are the enabled kernel backports a Debian stable default or an OMV default?

OMV default. It has been the OMV default for almost a decade.

Quote from prtigger1

Is there an easy way to uninstall all packages installed from backport repository, to get back to pure stable Debian basis?

OMV and omv-extras pin backports packages so just specific packages get installed not all backports packages.

OMV pins just the kernel and omv-extras pins these packages - https://github.com/OpenMediaVa…usr/sbin/omv-aptclean#L89

dpkg -l | grep '~bpo'

Will show you all backports packages.

Why the paranoia with backports packages?

Quote from prtigger1

The stabe kernel (6.1.x) is working perfectly for me....

You will have issue upgrading to OMV 8 then. The Debian 13 stable kernel is basically the Debian 12 backports kernel.

Quote from prtigger1

I would prefer a stable Debian basis with the option to switch backports to 'on'...

not a stable Debian with backported kernel with the option to switch backports 'off'

On a fresh install of OMV, you only get stable packages even though backports is enabled for the JUST the kernel. If you disable backports before installing any updates, you will get what you want. There are many more people who need the newer backports kernel than must have the stable kernel which is why backports for the kernel is enabled by default.