I keep thinking about this.
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Most retro handhelds do not have cellular network chips, gps, or even built it microphones or cameras in many cases. But many do still support wifi and Bluetooth.
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The vast majority of them do support either Linux, Android, or both. This is the area that needs the most work, since the Linux distros on these devices are so stripped down that they can't do much more than run emulators and a few bespoke game engine compilations. And for the Android-supporting devices, there would be a need to build more privacy-respecting roms. But that's the thing - many of these devices openly support that, it's just not something the communities have gotten around to creating.
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While this would become less useful with popularity, this kind of approach would be a form of steganography. If you're in an extreme situation where you or your belongings are being searched, how many people are going to suspect that the little Retroid Pocket gaming handheld is even something you can or might be storing your private info on?
Edit: Judging by the comments so far, I underestimated how unknown these devices must be still. While they do technically include handhelds like the PSP/Vita, 3/DS, etc; these days when people use the term "retro handheld" they're usually referring to a veritable cornucopia of gaming devices that come in a wide variety of hardware configurations and form factors. They are most often ARM-based devices, though there are even a couple that are pocketable fpga devices. Some of them are even small enough to be keychains.
Right now some of the most popular companies in this category include Retroid, Anbernic, Ayn, and Ayaneo. There is also a large selection of 3rd party custom firmwares out for many of these devices. But again, most of these are just very stripped down versions of Linux. Instead of full fledged desktop environments, they normally have media center style frontends like Emulation Station. And as far as I know, none of them have bothered to port any of the conventional Linux package managers.
As far as I understand, there is no technical reason why PostmarketOS, Mobian, or LineageOS for MicroG couldn't be ported to at least some of these devices, as some examples.
Hopefully that is enough resources for anyone to start to get up to speed. It should be apparent that full, unbroken system experiences with up-to-date software is possible on at least some of these devices, even including apps like Signal.
