It's not just missing from the context menu. It doesn't show up if I try to look for it in apps and features either.
This option has disappeared from the context-menu for ISO files, although the burner program is still there in Windows.
A workaround that doesn't involve any registry hacks is:
- Right-click on the file and select "Open With"
- Select "Choose another app"
- Select "More apps"
- Scroll to the bottom and select "Look for another app on this PC"
- Browse to folder
- Click Open.
Now you can go ahead and burn your ISO.