Officially, the common name is deprecated and no longer recommend for use by the Baseline Requirements.
Currently, Let's Encrypt still includes Common Names where possible. This is done for improved compatibility with old software. It automatically promotes one of your SANs as CN where applicable.
Unlike SANs, CNs are limited to 64 characters. If all of your SANs are longer than 64 characters, it is possible to receive a certificate without a Common Name: Simplifying Issuance for Very Long Domain Names
For client authors, the current recommendation is to not include a CN in the CSR. This lets the CA auto decide the best course of action.