Hi Julian!
Hello there,
I'm the maintainer for Anki, which is soon to release version 2.1.0.
It depends on Qt5, but they have decided to depend specifically on
version 5.9.x, because that is a long-term support release of Qt.
(See the start of http://doc.qt.io/qt-5/whatsnew59.html for example.)
Debian doesn't support 5.9 any longer, so do you have any suggestions?
Yes: don't worry at all. Qt is backwards compatible, so if upstream builds and runs against 5.9 it will continue doing do with any later version *except* they are using a bug as a feature, which would be a strange case.
But basically: relax and enjoy using Qt on Debian :-)
Cheers!