I was having this issue.
I figured out that apps are stacked in the taskbar by their AppUserModelID. Looks like very few programs specifically set a value for the process, window, and shortcut. Windows automagically gens an ID for programs that don’t specify a value.
I was able to find and get rid of the V6 one. Run “shell:appsfolder” to get a list of apps on your PC. Alt+V to show Details and then Choose Details. Add the AppUserModelId column.
Right click the Rhino 6 instance and Uninstall.
F5 to refresh the view and it should be gone.
Now I can pin Rhino and have the V7 one. Right-click now works for recent files as well.