If you need a reason, the modem and the baseband firmware have several unpatched vulnerabilities on that model, which is not something that an Android update can resolve.

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throwaway198846 an hour ago | parent [-]

Is there an easy way to tell if there are unpatched vulnerabilities in my phone's modem and baseband?

	▲	PaulCarrack 5 minutes ago \| parent [-]
		Based on the amount of modem CVEs posted in the monthly Android security bulletins, I think it's safe to assume that if you are no longer getting updates then there are unpatched vulnerabilities in them.