Earlier quoted context omitted.
How do you undo an app or subsystem after an update you don't like? For example I upgrade my IDE but not in place, I keep previous version just in case the new one is buggy or they again moved shit around. For my main system I am on LTS and I upgrade if there is a need and not to get high on version numbers. For example I tested new versions of kernels and video drivers and end up on what feels right for me and stopp…
Arch keeps a package cache of old packages, if you want to forever. So installing old packages is trivial (iff you had them installed before).
I don believe this is true for ANY package(like install an old KDE4 app on your KDE5 system)