The question may not be when was the system introduced but rather when was it discontinued?
The 2014 iMac was replaced and discontinued in 2015.
The 2015 MacBook was replaced and discontinued in 2016.
The 2015 MacBook Air was replaced and discontinued in 2017.
The 2014 Mac Mini was replaced and discontinued in 2018.
The 2015 MacBook Pro, while replaced in 2016, was only discontinued in 2018.
The 2013 Mac Pro was replaced and discontinued in 2019.
The 2017 iMac Pro was discontinued in 2021.
I see a pattern, and it is not based on hardware or other technical reasons. What I see is a 2017 cutoff. Macs that were discontinued pre 2017, aka more than four years ago, do not seem to get the official Monterey blessing.
The 2014 iMac was replaced and discontinued in 2015.
The 2015 MacBook was replaced and discontinued in 2016.
The 2015 MacBook Air was replaced and discontinued in 2017.
The 2014 Mac Mini was replaced and discontinued in 2018.
The 2015 MacBook Pro, while replaced in 2016, was only discontinued in 2018.
The 2013 Mac Pro was replaced and discontinued in 2019.
The 2017 iMac Pro was discontinued in 2021.
I see a pattern, and it is not based on hardware or other technical reasons. What I see is a 2017 cutoff. Macs that were discontinued pre 2017, aka more than four years ago, do not seem to get the official Monterey blessing.