I have some potential good news on this topic. My new freeware MSFS Livery Toolkit application can read KTX2 files and convert to PNG. While KTX2 to PNG conversion is not its primary purpose, it should solve the problem reported here.
At least from my testing so far, files which previously yielded the "KTX reader reported: Does not know about supercompression scheme 65536" in NVTT can be converted to PNG OK.
Plus a couple of other improvements over NVTT:
- It can open normal map KTX2 files without scrambling the colours into a yellow and black mess.
- It manages sRGB / Linear colour space correctly so there’s no need for manual corrections of overly bright textures.
The tool is still in a beta phase. If you run into a texture which it fails to convert correctly please open an issue on the Github repo so I can investigate and fix.