ST8 is not supported on Windows 10 because Windows 10 is not a released product, and thus is still a moving target to support. When it is released, look for a maintenance patch for ST8 to support Windows 10.
"Working" and "supported" are 2 different things. If you encounter an issue where ST8 doesn't completely work on Windows 10, it will not be fixed if it is not supported.
Now Siemens is expecting to support ST8 on Windows 10 with a maintenance patch, however the initially released version is not supported on Windows 10. Also, some folks have stated that ST7 with maintenance patch 4 (I believe) works on Windows 10, however it will never be supported on Windows 10, and no one can be certain that it works 100%.
I have been running ST8 on a new notebook since early November. Although I only use it on an occassional basis, it runs fine and I have never had any issues with Solid Edge running.