Hello Tim 
As of January 11, 2021, we’re planning to make a change to our API so that ACME clients will, by default, serve a certificate chain that leads to ISRG Root X1. However, it will also be possible to serve an alternate certificate chain for the same certificate that leads to DST Root X3 and offers broader compatibility. This is implemented via the ACME “alternate” link relation. This is supported by Certbot from version 1.6.0 onwards. If you use a different ACME client, please check your client’s documentation to see if the “alternate” link relation is supported.