From Engadget.com….
You might have to upgrade your Android phone before long if you want to visit large portions of the secure web. 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.