I need to install IE9 on Windows 10, but cannot. I keep getting a
message "Internet Explorer 9 is already installed on this system".
This is by design. You cannot run IE9 on Windows 8, Windows 8.1, or any version or future version/build of Windows 10 unless you use a virtual machine and install a version of Windows other then those versions on said virtual machine.
BTW, I support a web app that uses data islands. That's why I still
So use one of the trial virtual machines that Microsoft offers for FREE for exactly this purpose. If the virtual machine option isn't acceptable you will need to install Windows 7 on the machine and use that option instead.
I removed windows 11 and all installed updates for Internet Explorer,
but no luck.
The instructions you followed only work if a previous version of IE existed before you installed IE11 on said machine. In your case since IE11 is the only version of IE that will ever exist or can exist on Windows 10 and for that reason it is only possible to uninstall/disable IE11.