It is interesting to see a very specific number 2021 here...Wonder why they picked that year as a cutoff? Did they have any (relatively) recent technology implemented around that time to make the update easier and future proof?
I know that this kind of long commitment is painful for platforms because third parties won't do a shit for old machines and platforms usually don't have a good way to enforce them to update their problematic firmware or whatever.
Probably a certain class of chipset/ SoC that was used in a lot of machines sold that year? Also could be that in 2020, when schools were buying oodles of chromebooks during the pandemic:
a) Schools were already buying years old models that made them infeasible to continue supporting
b) ...they want all the schools that bought tons of chromebooks in 2020 to re-up
I know that this kind of long commitment is painful for platforms because third parties won't do a shit for old machines and platforms usually don't have a good way to enforce them to update their problematic firmware or whatever.