I have a little experience with this, designing systems that use TPMs and remote attestation to shut down communication rather than allow forgery or leaks. It’s very delicate to begin with, even if one entity owns all the devices, and unsolved for the general case.
But iOS and ChromeOS seem pretty good.
I'm referring to people casually saying: `Download Signal and you'll be fine` and not pointing out other OPSEC practices we all need to follow, regardless of threat model. (Not clicking on malicious links sent via SMS etc).
That doesn't really add up - a goals of things like more secure phones and secure general purpose messengers is explicitly to provide a high level of security without following a bunch of OPSEC practices. You aren't going to get mass adoption of more secure systems by insisting everyone take up OPSEC practices 'regardless of threat model'. The aim is to reach that adoption through systems that offer a lot of security without a lot of security-related ceremony. It's sort of inherent in the notion of mass adoption.
There is no such system. However, there are phones based on FLOSS, which you can trust and verify: GNU/Linux phones.