Debian 11, risky with this panel as with the last version published was to support Deb9, with some little to no-changes, you could go to Deb10 (which I'm using, still with LTS support, that's why I'm still using it), but Debian 11, there might be some changes to get a full support.
If by "Custom DNS", you mean the use of the "Add a custom DNS record" (https:// PANEL/client/dns_add.php), well, for me that I've both OpenDKIM and PHPSwitcher (and also Let'sEncrypt), I never experienced such an issue (and I changed a lot of time the PHP Version in use since the initial setup
So I'm afraid that I'll not be able to help you on your issue.
But, if you did modified some DNS listener file, this could be a bad implementation in those plugins, where the listener is not... listened (that's the only thing I can think of).
I only did the upgrade from Debian 9 to 10, never fully reinstalled it.
Now that some binary aren't available in the base repo of either "supported" distro, I would assume this to be quite difficult for a fresh install, but with some modification, this should be possible, need to be checked (you can try with a VM on your PC if you want).
You seems really mad... if you are not happy reitbrud , just leave with your rage.
I've all the plugins you cited here, all still work flawlessly on my setup... (including latest versions of PHP).
You switch over to another system ? nice for you, so why you keep posting there ?
Well, I guess a "simple way" (it's never simple) would be to remove all the base PHP versions installed (which you overwrite by updating the 7.1 needed by the panel), and then rerun the imscp setup.
This way, the panel will reconfigure itself with the correct version.
Maybe even with some parameters, this setup can be launched just to reconfigure the PHP part (but I'm too tired, might be noted somewhere in the wiki ).
And yeah, PhpSwitcher make some stuff way easier, I'm using it with all the latest version of Php (8.2.8 right now) :
Having this posted twice will probably not get you more answers (Initial post : extra PHP 8.2 installed on server - frontend/rainloop now seems to be using this and get errors)
Anyways, this is an unusual setup and, as far as I'm aware, not supported in any ways.
If your goal was to have one or more subsites/domains to use a different Php Version, the (payed) plugin PHPSwitcher would have been the way to go.
So far, I didn't get time to look for another solution, and as far as I'm concerned, we can run it with Debian 10 (with a few easy tweaks), so right now, I set the deadline to June 2024
For now, I didn't see a big "security concern", I still keep a close eye to my server, but it's running smoothly (and I've multiple backups anyways, as anyone should have), I'll continue to support you and the project.
Glad you found some people willing to invest time on it (and that you can trust their work too ! ).