Windows 8.1 reduces the available screen space when you bring up the touch keyboard and dock it to the bottom (accordingly, querying with SPI_GETWORKAREA returns a smaller area). Application windows retain their maximized state but get smaller in height so that the entire window remains visible. That makes a lot of sense. Windows 10 doesn't do that. Bring up the touch keyboard, dock it, and it will overlap any maximized windows and possibly obscure the very area you are trying to type into so you can't see where or what you're actually typing. That is utterly ridiculous. Toggling tablet mode doesn't change that behavior. Any ideas?
Have you try to use the Windows 8.1 (or 8) Driver? I would try that, just would set the installer to Windows 8/8.1 Compatibility Mode for 8/8.1 first!
AFAIK, such software still works with drivers (and special settings). I still would set Tabtip.exe to Windows 8/8.1 Compatibility Mode. Anyway, it's simply up to you what to do!