Hi Zdenek,
a QA person here. This sounds like it's going to be a major headache for us (and our users), I hope I'm wrong. I initially skipped your message, because it was too technical (seemed targeted at system admins, not regular users) and I don't know half of the printer-related abbreviations and terms. I guess my blissful ignorance ends :-)

I very much like how Chris summarized the problem in a more user-friendly language in test list [1]:
The very rudimentary summary is:
1. When upgrading (does not apply to clean installs);
2. with a printer that supports ipp-usb (a.k.a. driverless printing);
3. using the native driver (which can be a cups filter, free or nonfree)
Printing breaks.

I believe this is something that was missing from your announcement.

I tried to create a Common Issues entry for our users here:
https://ask.fedoraproject.org/t/common-issues/20975

Please help me finish it by directly editing or suggesting additions and fixes in comments. That text should be readable and actionable by an average Joe, deep technical details can be linked. This is all just printer-related, scanners didn't fit into my brain for the moment, they'll get a separate treatment.

This looks like a major change, I wonder why it wasn't included in F36 ChangeSet [2]. I think we should consider adding it there, even though it's way past the deadline. It would make the change more visible/searchable and also make the description and workarounds more accessible.

Now, I have a load of questions:
1. Is Chris' summary above correct?
2. Does this affect only USB printers, and no network printers?
3. Can you estimate what portion of our user base (who own printers) is going to be affected by this? How common are printers supporting IPP over USB?
4. If the printer doesn't support IPP over USB, what will happen? Will the printer continue to work as usual, and the ipp-usb package will not interfere?
5. How can an average Joe tell whether he's using a classic driver (which is incompatible with ipp-usb)?
6. When you talk about 'removing the old print queue', is it the same as removing the printer from system settings (e.g. gnome-control-center)?
7. If Joe removes the printer from system settings, what will happen then? Is a reboot necessary? Will the printer magically appear there by some autodiscovery? Or is it necessary to manually add the printer, but no driver selection is needed? Alternatively, is it possible that the printer will only appear in print dialogs (from different apps), but it will not be listed in system settings?
8. Is it necessary that Joe also removes the real driver from the system (like hplip), or will the action described above be sufficient?
9. I read that Firefox might not work with the new setup [3][4]. I'm *very* concerned about that. Can you elaborate? When exactly will printing from Firefox not work? For all IPP over USB printers handled through ipp-usb?
10. Can it happen that the IPP-over-USB approach offers less printing options than its real driver counterpart? E.g. paper types, color adjustments, etc. What if the user wants to use the real driver instead, for these reasons, what is the recommended approach? (Ideally for an average Joe, if possible, i.e. no lpadmin commands).
11. What can we do better during the upgrade? I read we can't fix this perfectly. But even if the package removed all "print queues" during installation, it would go from "My printer doesn't print and I have idea why, I'm so angry" situation into "My printer disappeared, I had to add it again, I'm so angry" situation (in case it wasn't IPP over USB, in which case it would be autodetected and immediately re-appear). That seems like an obvious lesser evil. In the first case, you have no idea what to do, except for magically stumbling on our documentation. In the second case, it's obvious that you need to add the printer again, if it is not there, and so it allows users to fix the situation themselves pretty naturally. I understand this won't work on rpm-ostree based systems, but it's still a huge leap forward. Am I misunderstanding something?
12. This can still be reverted, right? It's enough to stop recommending ipp-usb in F36, correct? Or is there a technical reason why that mustn't be changed? I simply wonder whether we still have a way out if this is deemed too catastrophic without some automatic workarounds like the one proposed above.

Thanks!
Kamil

[1] https://lists.fedoraproject.org/archives/list/test@lists.fedoraproject.org/thread/YL3XCMM7O27MEG6B2K54L2YSP2OJ7ZJ4/
[2] https://fedoraproject.org/wiki/Releases/36/ChangeSet
[3] https://bugzilla.redhat.com/show_bug.cgi?id=2066528#c4
[4] https://bugzilla.redhat.com/show_bug.cgi?id=1983403