As long as you still have access to the cli it should be fixable. If you want to still try to get to plasma 6 make sure you also enabled the core-testing and extra-testing repos in addition to kde-unstable as per the wiki

If you enable any other testing repository listed in the following subsections, you must also enable both core-testing and extra-testing

I missed that little snippet when I first swapped over.

If you do yay kf6 you can install all of the framework-related packages which might also help fill out some missing dependencies. For me it's 1-71. You can do the same with yay plasma and then choose the ones from kde-unstable (122-194 for me) but you will have to manually avoid the ones with conflicts like plasma-framework.

But if you want to try and revert theoretically simply removing the testing and unstable repos and doing another sudo pacman -Syu should get you back onto the older versions.