How can you set Android to never update the operating system?
(What I've noticed is that the OS updates oftentimes cause older Android devices to slow down.)
I do not think it is really possible to "Stop Updates" or that it should "never update" without rooting etc.
But... since Android is never Auto Updating (installing the update without you touching anything), you are essentially just asking on how to remove the nagging notification/s, am I right? That should be easily done (as with any app's notifications) when you get the notification ling press on it and it should give you an option to disable it.
Would upgrading a Moto X4 to Android 9.0 Pie result in the device being slower than not upgrading to Pie?
Because the newer versions of Android may be more hardware resource intensive.
No one can really answer you this. It is not a design thing but rather that sometimes updates introduce bugs and deficiencies that the old firmware didn't have. It can even vary from one user to the other depending on their settings and what apps they have installed (something bugs like this can be becuase of a spesific setting/app not playing nice with the new uodate.)