[pki]

then we get into the whole "at what point do we stop supporting windows xp for real this time guys" argument though..

[AdmiralAsshat]

There's a big difference between a 10 year-old OS and a two-year old phone, though. Especially if the unlocked model of the phone has received a Marshmallow update, but your carrier decides they don't feel like going through the cert process for that when they could easily twist your arm to getting a nice, new $750 phone instead.

[JohnTHaller]

For Google Chrome the answer is: April 19

[fixermark]

That decision is generally made by the app vendor. Apps in the Play Store declare a minimum OS version they compile / run against.