I’ve burned Windows 10 iso to dvd disk then run the installer from dvd drive on my old Windows 8.1, I was not intending to perform an upgrade, I just wanted to make sure that that the installer would run fine. After exiting there was a folder named $Windows~BT created under root directory of disk C (C:\$Windows~BT). I manually deleted that folder and now Windows Update lists MSRT for Windows 10 among offered updates
Should I remove some leftover registry entries as well to stop windows update offering updates for Windows 10? I don’t know where to look for those entries in regedit.