It is a somewhat newish feature, I think pre-2020 models don't have dark mode, afaik (and I have no idea how are old devices supported on kindle, I assume launch and forget, could be wrong though).
It seems Amazon has financial and legal incentive to keep older Kindles (it is so many of them still) updated to avoid them getting jailbroken and not having Kindle store at their fingertips :)
Not sure if they cut it off at some point, but they can surely keep updating 10 yo devices.
