That only matters for parts and service. In terms of OS support, it’s always based on launch date. macOS Sonoma for example requires iMac 2019 or newer.
True, but Big Sur from 2020 is still getting security updates until 2023, which is compatible with a number of older machines, so Macs can still run a supported Mac OS for at least 5 years. For a 2019 Mac, even if you assume Sonoma is the last OS it supports, with the typical 2 year security support, after an OS is discontinued, that Mac will have reached vintage status before it all support for its OS ends.
My point was vintage pretty much decides when hardware and OS support, for an OS a vintage machine can run, ends. Although even some obsoletes machines, in terms of hardware, still can run a supported OS in 2023. So it's a bit more complicated in terms of when support ends than just intro date, in terms of how long a computer can be repaired and or run a supported OS.