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I know for a fact that it supports end-to-end encryption on alternative operating systems. They don't do anything to disable it. It's easy to check this, and not that hard to actually verify it at a deeper level. They're just washing their hands of supporting other OSes.
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They're just being lazy. They aren't going to make people feel insecure by pointing out that most mobile devices have trash security. If they warned people about having an out-of-date patch level, I'd be more sympathetic, because third party OSes generally lie about patch level.
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They aren't disabling end-to-end encryption on these operating systems. They're just trying to absolve themselves of having any responsibility to deal with it. If they started warning people about having explicitly insecure devices without even basic security updates, I'd get it.
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