Supporting older hardware is extra work that doesn’t bring in extra money. Also, oftentimes, it isn’t possible to backport features in a performant way (a lot of the ML stuff would only crawl on 10 year old hardware, features such as Handoff and PowerNap require hardware features). End result would be a 20 year old machine that runs the OS, but doesn’t work with modern software.
That wouldn’t make customers happy, and would dilute the brand of their OS releases.
Mojave for example dropped all Macs with GPUs incompatible with their Metal API.
https://arstechnica.com/features/2018/09/macos-10-14-mojave-...
The arstechnica MacOS reviews are good for working out (sometimes resorting to speculation) what makes a Mac unsupported.
To my knowledge Linux has never worked well on intel macs with a T2 chip. Asahi linux is working on bringing good support to m1 macs, so it looks like for good linux support you either need a pre-T2 mac or a post-M1 mac.
For the more common and popular hardware there is a good chance that open source drivers can be maintained by the community but if your laptop relies on a somewhat obscure chipset or microcontroller then your mileage will vary...a lot. Look up "Intel GMA500 Linux driver" if you need an example of the pain.
Sometimes the decision could be entirely commercial. Most notably, OSX dropped support for all nVidia GPUs from Mojave onwards despite nVidia going on record saying they are happy to continue providing drivers but Apple won't sign them.
Not those shipped with Macs. The GeForce kexts to support the NVIDIA GPU gens that Apple shipped, Fermi and Kepler, are still present even on Monterey.
https://github.com/chris1111/Geforce-Kepler-patcher
Fermi was never supported beyond High Sierra IIRC.
TIL that support for NV cards on Monterey is gone, it definitely was there in the betas.