It seems that Qt5PlatformSupport was modular and thus broke into several libraries.
I found this Qt code review entry , which shows that instead of defining a single QtPlatformSupport target, the QtPlatformSupport template is used in the project file. I also found this KDE code review entry where you can see that they use Qt5PlatformSupport prior to Qt version 5.8.0 and Qt5FontDatabaseSupport , Qt5ThemeSupport and Qt5EventDispatcherSupport after that.
The specific new libraries you need for your project are likely to be different.
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