All devices launched September 2011 and forward. I agree that they are missing a lot of previous devices, but there are already tools out there to unlock the bootloader on those devices.
If I were to make a guess, I would say the September 2011 mark is all devices that are using the current bootloader locking method, and so they are just supporting that instead of going through and making other programs for all previous versions.
There are no simple unlock tools for some of the S-ON phones launched before September 2011 (with HBOOT 2.0). HBOOT can be downgraded[1], but it's quite complicated.
If I were to make a guess, I would say the September 2011 mark is all devices that are using the current bootloader locking method, and so they are just supporting that instead of going through and making other programs for all previous versions.