You shouldn't measure by the release date of the hardware, but by the last sale date of the hardware. In Apple's list of supported Macs, the "iMac Pro (2017)"

You shouldn't measure by the release date of the hardware, but by the last sale date of the hardware. In Apple's list of supported Macs, the "iMac Pro (2017)" is the oldest year. But that computer was sold until 2021! Makes Apple look more generous putting 2017 in that list instead of 2021!