I am trying to find out whether there is a guaranteed way to continue obtaining leaf certificates that can be verified against the DST root until 9/2021 without using alternate link relations when downloading the certificates (but with use of manually overridden intermediates.) Our ACME client library currently doesn't support alternatives, so it's useful to know whether or not we really need to do anything about that.
One change LE could potentially make that would require the ability to download alternate leafs would be if the leaf certs provided by default started being signed by E1, since that chain doesn't have a cross-sign. This forum post mentions that Let's Encrypt may do exactly that, but I can't find a more authoritative source for the transition to E1 schedule. Are there plans to use E1 as the default, either on January 11 or before September 2021? Thanks!