Hi there,
Yes, only 8th generation processors are supported for Windows 11. This is been specified by the PC manufacturers for improved security and performance. You can also install Windows 11 on an old PC via clean install, but you might not get any updates.
This link might be helpful with steps https://answers.microsoft.com/en-us/insider/forum/all/how-to-perform-a-clean-install-or-reinstall-of/aef0ae63-2117-41ee-a8ea-4a3181625b08
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