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 stopped there. Sometimes the new video driver is more unstable then the older ones so I would never do a driver update without having a good reason and time to evaluate it.
Like just a few days, weeks or continue to use a older no longer maintained version of a package because you don't like the new version.
I (now) guess you mean the later in which case, yes it's not trivial as it's not really supported by anyone anywhere. Neither the original software developers (which replaced it with a newer version), the package maintainers/Linux distro (which also moved on as the older version doesn't get updates), other packages interacting with ti (which expect a newer version), etc.
So while it might suck, I would recommend to simply not do so as it's not worth the risk and headaches it brings you. At least not as long as you don't find enough people to do a fork and maintain that fork.
Still undoing you last update is often as simple as just installing the package from the cache, and then you would need to pin it/make pacman ignore it (and then it probably will brake sooner or later depending how much it relies on specific system libraries in specific versions being available).
I disagree, when you release a new version of something distros like Debian or Ubuntu will backport the security fixes but not backport the new features and the new bugs.
As an example we are a group of people with some eye disabilities but not completely blind, we use an old application Jovie for KDE, this app was removed from latest KDE and replaced with something with lot less feature. We are compiling this old app from source and at least for this old LTS distros we can get both Qt4/KDE4 and Qt5/KDE5 libs installed. So imagine people with disabilities doing an update one day and poof your must have thing is gone and you can't use your system anymore and some person on the internet will tell you that you are doing it wrong and should not use anythng else then the latest version of stuff because the developers do not support old stuff.
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 stopped there. Sometimes the new video driver is more unstable then the older ones so I would never do a driver update without having a good reason and time to evaluate it.