Might as well say it since nobody else commented about it, but modem/soc vendors are huge limiting factor on longterm android support. Qualcomm maintains these updates for only a few years, basically nothing earlier than around 2020-2021 gets kernel driver or modem updates.
Of course it's still up to phone manufacturer to integrate these changes, but it puts an effective security support timeline on even 3rd party ROM's like lineageos. They can cherrypick, but it's not as secure once that support ends.
Apple has almost everything in-house (except until recently, modems). So they have a ton of flexibility in continuing to provide updates.
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