A colleague asked for me on the mailing list and that was the answer:

regarding the 'deprecated' state of XmlPatterns: There wont be an further development for this module, but I also doubt that it will be removed before Qt 6. So as long as you're using Qt 5 you're safe to use XmlPatterns. Qt 6 might be still 2 years away, and as you're using it professionally, I assume that you'll probably wait at least till the release of Qt 6.1 (or even longer) anyway before upgrading your project to it. Until then there might be another Qt (conforming) solution to the problem.

So in my opinion you've got 2 options (depending on the scope and lifecycle of your project):
a) use the Xml Schema related classes from the XmlPatterns module and worry about it going away (much) later, and maybe even have a Qt (conforming) solution by then, or
b) use an external library like CodeSynthesis XSD or something similar and worry about their API changes and usage and naming patterns that differ from Qt's patterns etc.

IMHO using the Qt modules while they are still available is usually the better option.

[[by Yves Maurischat]]

I hope that helps other developers with similar questions. I mark this as solved in the sense that there is some sort of answer here, although maybe it's not completely satisfying.