As somebody often connecting to networks not belonging to me (legally!), I have a variety of VPN clients. It seems that everybody uses something else. That's why I had to install FortiClient a few months back. If you never heard of it, it's a VPN client that thinks it's more than that and has no support for VPN portion under Linux.
While FortiClient wasn't the worst VPN client I ever used, I must say there were no tears when I didn't need it anymore. As any normal person would, I went to uninstall it only to be faced with a grayed out Uninstall button. Never mind - Windows 10 Settings app might be buggy - so I went to the Control Panel. There situation was even worse as I only had Repair available. No uninstall again.
I searched Internet for a solution and found knowledge base article by Forti itself. And it describes the exact procedure I tried to no avail. So I searched a bit and found solution on Reddit of all places (might be the first time I ever used Reddit for anything other than wasting time).
Solution was to use wmic in order to trigger uninstall. For this one should write the following into the Command Prompt with administrator rights:
Admin Command Promptwmic product where "name like 'Forti%%'" call uninstall /nointeractive
This will uninstall FortiClient and reboot computer automatically afterward. And finally it's gone.