The past few times I’ve run yay I’ve got these warnings about packages that are orphaned/not in the AUR. Based on the names I’m assuming these are leftover from the upgrade from kde plasma 5 to 6, are these safe to remove now? And secondly how would I find orphaned packages like that if I wasn’t using yay since I never installed these from the AUR?
Define safe. Your can recover from just about anything with a live USB by just reinstalling any missing packages. But that is only needed if you remove some vital packages, those in base or to do with your bootloader. Which is only a small list. And none of those will be in the AUR unless you explicitly opted into something critical (like zfs as your main filesystem, or a custom kernel).
Most things, especially GUI stuff is optional in Arch. You might break your GUI but you should always have access to the CLI to reinstall things like you did when you first set things up.
The only time I would be really wary is if trying to remove a package also tries to remove half your system packages. Then it is more likely to be required by something you want and should very carefully audit the list of packages it wants to remove.