Recently, the Let's Encrypt root certificate has expired. This is one of the first major digital certificates to expire since the advent of the internet. Therefore, there is no precedent for how to solve the problem besides updating the software on devices. In normal circumstances this event, a root CA expiring, wouldn't even be worth talking about because the transition from an old root certificate to a new root certificate is completely transparent. The reason we're having a problem at all is because clients don't get updated regularly and if the client doesn't get updated, then the new root CA that replaces the old, expiring root CA is not downloaded onto the device. One of the notable clients that will still be affected by this expiration is anything depending on the OpenSSL 1.0.2 or earlier library, release 22nd January 2015 and last update as OpenSSL 1.0.2u on 20th December 2019. These are some of clients that will have issues
OpenSSL 1.0.2 Windows XP SP3 macOS 10.12.1 iOS 10 (iPhone 5 is the lowest model that can get to iOS 10) Android 7.1.1 (but >= 2.3.6 will work if served ISRG Root X1 cross-sign) Mozilla Firefox 50 Ubuntu 16.04 Debian 8 Java 8 8u141 Java 7 7u151 NSS 3.26 Amazon FireOS (Silk Browser)