It's likely going to be recent devices. Older devices Android was tightly coupled to kernel version, didn't have a good hardware abstraction layer for drivers,

It's likely going to be recent devices. Older devices Android was tightly coupled to kernel version, didn't have a good hardware abstraction layer for drivers, etc. Basically android has been working on restructing itself under the hood over the years to be more maintainable. Further, older devices will be on LTS kernels which will already be outside their support window, so there's a ton work to be done updating the device tree to upgrade to a newer LTS kernel. Short windows made sense simply because it was a lot of work to maintain old devices. Now that there's a more solid separation of concerns, longer support windows make sense.