The primary focus of Qt's high-DPI scaling has been for OS X and iOS Support with offering a scale factor for scaling the graphics output., which works the best when using vector graphics and fonts. Now that the Apple support is in good shape, other platforms are being looked at by Qt developers.
For Linux users, Qt developers are looking at using Wayland's wl_output::scale value that was added to Wayland 1.2. Qt developers have already played around with this HiDPI Wayland support. Developers are also working on platform-independent high DPI scaling support by adding a coordinate translation layer to the QWindow and QWindowSystemInterface classes.
Those interested in more information on the HiDPI support for Qt should read this mailing list post.