Economics 101 tells us that for-profit companies (across the board) do their best to get people to buy more things from them. Tech companies are notorious for forcing you to upgrade, but other companies also do their best to make things obsolete so you will upgrade and buy more from them. That's just how companies make money. That's why, planned obsolescence is here to stay. In fact, one could argue that it planned obsolescence is good for investors, but not for consumers.
My gut feeling is that all apple products, with prioretization on their highest volume (Ie, iPhones) have built in planned obsolescence. Here are some cases.
Case 1: iMac 2009. Top model at the time, 4gb of RAM, 26”. Machine was crawling slow by 2018, so bad that I had to buy a replacement iMac. Why do things that used to work fine now take forever? New OSes are heavier on purpose. Also: Died 2018. And when I say died, one day it wouldn’t boot. Period. I’m a computer scientist, I did try tinkering with it for a couple of days, got it to boot in Linux mode only but never got it to run properly. Trashed.
Case 2: MacBook Air 2008. Died 2018. That was even worse, I couldn’t get it to boot at all. And yes it was also dog slow by that time.
Case 3: iPad 2008. Became unusable by 2013, too slow for anything.
Case 4: iPhone 7, 2015. I’ve kept it on as old an OS as I can, it can work with very limited functionality (I use it as an iPod and had to buy a newer phone)
Case 5: iMac mini 2008. Crawling now. I had to update to the latest OS because the Apple remote wouldn’t work with older versions. I can tell it’s about to die. It really had started becoming annoyingly slow around 2014, I just keep delaying its replacement out of pure annoyance at Apple’s planned obsolescence.
So the verdict is that the new OS versions overwhelm the old hardware. And to top it off when old Macs die, they die suddenly. Keep backups. Except if you use a time machine expect the Mac to be even slower and to die faster. (Catch 22 a-la-Cupertino).
All hardware & software manufacturers do this, and Microsoft used to be the pioneer. After about 7 years the stuff isn’t working anywhere near as well as it used to. If you’ve bought a top of the line model you may get a few more years out of it, otherwise the 7 year itch will hit hard.
How do you keep your Mac from planned obsolescence?
You can’t. It will become obsolete eventually.
Even if it works perfectly fine for years Apple and all developers will eventually stop releasing updates and software for the last supported OS and x86 architecture.
Does Apple practice artificial obsolescence with Macs as they do with their mobile devices?
Yes, however they are not alone - just really, really organized. Consider two elements of the Mac:
Hardware. Parts become hard to find. Many parts that on different models used to be socketed are either a part of the board or soldered into the board. Sometimes, simple repairs invalidate your warranty
Operating System Updates. New updates can often include quite non-critical changes that totally harm performance. Once Apple stops updating the OS for your computer, you have serious problems. They can also drop key developer libraries needed by third party apps.