Back in April 2019 I wrote
Let's Encrypt to transition to ISRG root, where Let's Encrypt had planned to move away from the IdenTrust root to their own root, ISRG Root X1, that expires on 4th June 2035, giving us quite a number of years. The problem was, not many devices had received the necessary updates that include this new ISRG Root X1, issued 4 years prior in 2015! If a large selection of devices had not received an update to include this new root certificate, they simply won't trust it. This is basically the same problem we're experiencing now with the IdenTrust root expiring, because client devices haven't been updated, they also haven't received the new ISRG Root X1.