I didn’t notice anything about this on my side, but I don’t use them to manage real-time actions…
It hasn’t been fixed?
For this, it will depend.
If user use appimage, everything you need for a cross-platform application is embedded in appimage (Qt fonts and QtSvg library)
After, if you use explicitely fonts provided with windows, most of them are under license and can’t be (legally) found on Linux… If you use unicode character, normally practically most of them are available in Linux (on my side, never has any problem)
If user use a version from a distro repository, it will depend of the distro and the maintener of package in repository; normally (on Debian for example) when you provide a .deb package to deploy an application, the package list all needed dependencies.
For Krita for example, it should list all Qt+PyQt libraries (module + version) that are expected for Krita (5.12.9 for Krita 4.4.1)
That’s easier to get a Linux computer than a Windows computer or even an Apple computer 
On my side, I don’t have Windows 10 at home, and won’t buy a license just to check if my plugin is working on an OS I won’t use 
I don’t even talk about apple, for which you have to sell an arm to be able to buy a more closed system than windows is…
On the other side, anybody is able to download a Linux distro and install it for free 
Grum999