Any method I can think of to replace UI assets would be prone to breaking with every patch Apple pushes out. I’m not going to say it’s impossible, but it’s probably not worth your time. If you want to explore what the UI felt like, you’re better off exploring VMs or old hardware and install media.

To be honest, with the way Apple does support for releases 1 or 2 back from current, you could potentially do this easier nowadays: just don't support the current version (e.g Monterey), only target Catalina or Big Sur. When Monterey is no longer the new one (only receiving security patches), that's when you roll it forward.