Unless you have special needs (needing to support older OpenSSL/LibreSSL/GnuTLS/embeddedTLS versions), nothing. (OpenSSL Client Compatibility Changes for Let’s Encrypt Certificates)
Root CA's do not sign subscriber certificates ("leafs"). Roots sign intermediate certificates which in turn sign leafs.
The certificate chain currently in use is:
End-entity certificate ("leaf") ← R3 ← ISRG Root X1 ← DST Root CA X3
This chain will continue to be used, even after DST Root CA X3 has expired. This ensures Android compatibility, but will break some clients named above.
You can request an alternate chain*, terminating at ISRG Root X1 (instead of DST Root CA X3), but this will cause problems with Android versions < 7.1 (and will fix other clients).
*How to do this depends on your ACME client. Some modern clients have an option, usually called preferred-chain.