System functionality could be expanded through various means, but most often devs used Extensions. And if a software issue arose, it was easy to disable all Extensions by pressing SHIFT key on start-up. Also, on start-up you'd visually see what Extensions were being loaded. So at start-up you would always be aware what you had installed.
In current macOS it's very hard to keep track of what I've installed. I install a lot of stuff using tools like Homebrew. Some software might install some system level hooks, etc... From my perspective it's kinda hard to keep the system "clean". And it's probably a good idea to do a clean install of my computers maybe once every year or so, since I might have installed stuff I don't really use anymore.
Also, there was the System Folder and that directory contains the Extensions, Preferences, Control Panels directories, etc... So you could also at the file system level manage your System Folder. You could just delete an Extension manually from Extensions directory in the System Folder to uninstall it. You didn't need any "uninstall" software most of the time.
A classic macOS like environment with a view more modern features (maybe a WindowMaker-like UI, multi-user and real multi-tasking) would be pretty neat.