Your older Android device will still be able to browse websites starting next year after Let's Encrypt found a way for these devices to be able to visit sites that use its certificates after September 2021.
While a third of existing Android smartphones were seeing error from websites secured by the certificate authority, it appears that a solution has been found for the issue.
Starting September 1, 2021, millions of Android phones running Android 7.1 Nougat from 2016 or earlier would not be able to connect to websites using Secure-Socket Layer (SSL)/Transport Layer Security (TLS) certificates from Let's Encrypt.
The main problem was that Let's Encrypt's original root certificate was based on a cross-signature from the certificate authority IdenTrust, which issued the "DST Root X3", which is set to expire on September 1, 2021.
Let's Encrypt now has its own root certificate, ISRG Root X1, but there was concern that Android versions prior to 7.1.1 did not trust ISRG Root X1 . And because there are so many Android devices running versions prior to that, the situation could have translated into much less access to websites that rely on Let's Encrypt digital certificates to provide HTTPS connections.
The new cross-sign extends beyond the expiration of the DST Root CA X3. The workaround should work for Android because Android does not enforce expiration dates on certificates used as "trust anchors."
Let's Encrypt explains that the "self-signed certificate" representing the DST Root CA X3 keypair still expires, but the browser and OS root contain "trust anchors" and Android is designed to ignore the anchor that sets a date after which it should not be trusted.
