And because the Qt print dialog in Manjaro does not list driverless printers, it is impossible to print on these printers from Qt/KDE applications. This is a quite important class of printers: in office setup many large printers only provide driverless printing (at least for linux).
From non Qt applications (firefox, gimp, libreoffice), it is perfectly possible to print on driverless printers in Manjaro. Furthermore, driverless printers get correctly listed in the KDE System Settings, so this seems to be more of a Qt issue than a cups configuration problem.
There is a workaround for this issue, but it is very much of a burden. From Qt applications, you print to PDF and then you use a non-qt PDF viewer to print to the printer.
The issue is not present in Ubuntu where the Qt print dialog perfectly lists driverless printers.
If this is a regression in Manjaro, then it is not a regression from the latest stable update. I have managed testing a system that had not received it and it shows the same issue.
I strongly suspect that Qt in Arch/Manjaro misses some important patch or is misconfigured in this regards.
I would be thankful if someone could test this on their Manjaro machines, particularly if they are on unstable and working with a version of Qt5 fully aligned to Arch, as this is an issue that Manjaro might be inheriting from Arch and that might need to be reported there. Obviously testing on Arch would be equally useful.
Note that to test you need to activate the cups-browsed service that is disabled by default in Manjaro.