On a Samsung S6 Plus Edge running on Android 7, certain security certificates will break since September 2021: the workaround which Let's Encrypt implemented doesn't seem to work in Samsung Internet. I get the standard "Back to safety" screen.
For browsing this is not an issue, I can use firefox. But I have an issue with WhatsApp: it will not load preview images from such websites, as probably it routes such requests through Samsung Internet or some samsung API. Changing the default browser doesn't help.
Is there a workaround for this which doesn't involve rooting the device ? I've seen this answer, but it looks like I can't install certificates manually if the device isn't rooted.
doesn't seem to work in Samsung Internet- it is more "does not work in any app except Firefox as Firefox has it's own certificate trust store". As your device does not receives updates the only alternatives are rooting + installing the Let's Encrypt certificate or install a custom ROM with a newer Android version.Samsung as the culpritI agree with you, that is the most likely reason, too many modifications by Samsung but the relevant they missed or did something wrong.