This page on Let’s Encrpt’s website documents what systems Let’s Encrypt’s certificates are and are not compatible with: https://letsencrypt.org/docs/certificate-compatibility/
The page currently says:
The main determining factor for whether a platform can validate Let’s Encrypt certificates is whether that platform includes IdenTrust’s DST Root X3 certificate in its trust store.
Since Let’s Encrypt is transitioning to ISRG’s root, this will no longer be true in the near future.
Could someone at Let’s Encrypt please update that page to indicate what clients will and will not be compatible with Let’s Encrypt once the transition to ISRG’s roots is complete? The existing compatibility information for IdenTrust’s roots should also be retained until such a time as using those roots is no longer an option.