What is the current behaviour and why should it be changed?

"the minimum supported Qt version, which is currently 5.9"
This has changed under #3288. The absolute minimum is now 5.12.2.

Describe possible approaches

1. CONTRIBUTING.md must be updated
2. The release notice should also make the dependency change clear
3. I guess the wiki needs updating, too -- so raise a separate PR for that linking to this issue

Has this feature been discussed and generally agreed?

Yep.