Answer: Basically forever.
Google is playing the game of "See, we got everyone in mobile to think 3 years of updates is good. Maybe we can do the same with laptops?"
10 years is good, but frankly for a laptop built on linux it's an embarrassment. If google would instead support a standard distro like debian we'd see way better support. (but then, they couldn't track everything as easily).
[1] https://learn.microsoft.com/en-us/lifecycle/products/windows...
[2] https://blogs.windows.com/windows-insider/2021/08/27/update-...
Apple was dropping support for specific Intel microarchitectures with each release for a while there.
Now that the future is ARM, these machines will be supported for a long time. Not sure how long the Intel Macbook Pros from 2020 will be. I bet it's not another 8 years.
2016 is an important cutoff because it is when they introduced the T1 boot security chip. Some future macOS will drop support for everything before these machines, which will permanently evaporate the concept of a hackintosh.
After double checking to see what's going on with old hardware, it became really clear that apple doesn't support their older laptops.