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Commodore 64's New Flip Phone Skips Android for Linux-Based Sailfish OS
(feed.itsfoss.com)
This is a most excellent place for technology news and articles.
You sure you didn't misread? To me it looks like they allow sideloading Email apps but disallow sideloading web browsers.
How are they determining if an an app is a browser?
A hard-coded list of known browsers would almost certainly work. There aren't that many of them. Or better, a list of the common support files that get installed alongside the main browser executable(s), so you can't just rename the main one and have it work.
Anything cleverer than that is about as technical as removing the custom OS and installing standard Sailfish.