Funny to read that they are stuck with an old Qt version, 5.12 to be specific. I am on a similar boat, stuck with Qt 5.11 as this ist the last one with support for macOS 10.11.
And I am stuck on macOS 10.11 because it's the last with proper PDF subpixel rendering in Preview.app.
However, many Qt developers seem to upgrade to the newest Qt version as soon as possible. If it's an open source software, I can self-compile it using the older Qt 5.11, which usually works. But by default, Qt is sadly not providing platform independency anymore.
I see no reason to support OSX 10.11 anymore, just like I don't see any reason to support Windows 7 anymore. We have stay on Qt 5.12 because we haven't got the time/are too lazy to fix the bugs in later versions of Qt that break Krita, especially on Windows.
And I am stuck on macOS 10.11 because it's the last with proper PDF subpixel rendering in Preview.app.
However, many Qt developers seem to upgrade to the newest Qt version as soon as possible. If it's an open source software, I can self-compile it using the older Qt 5.11, which usually works. But by default, Qt is sadly not providing platform independency anymore.