According to Android Police, Certificate Authority Let’s Encrypt is warning that phones running Android versions before 7.1.1 Nougat won’t trust its root certificate starting in 2021, locking them out of many secure websites. The organization will stop default cross-signing for the certificate that enables this functionality on January 11th, 2021, and will drop the cross-signing partnership entirely on September 1st of that year.
You can try to install Firefox and use its own certificate store, but that won’t help with rival clients or functionality beyond browsers.
About 33.8 percent of Android users on Google Play run a version older than 7.1.
This is that happen if you have literal monopoly on free certificates.
We’re happy to announce that we have developed a way for older Android devices to retain their ability to visit sites that use Let’s Encrypt certificates after our cross-signed intermediates expire. We are no longer planning any changes in January that may cause compatibility issues for Let’s Encrypt subscribers.
Criminals backed off, but they will return soon.
https://letsencrypt.org/2020/12/21/extending-android-compatibility.html
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