The renewal emails were for a service, that was once provided by let’s encrypt, to notify you if your certificate was about to expire (see link above). There was really no reason for that test for LETSENCRYPT_ACCOUNT_EMAIL to be included when it was added 6 months ago.
I think that originally (8 years ago or so) Let’s Encrypt required an email address, which is why the test for it ever existed. For a long time, if you didn’t supply an email address, they’d not give you a cert.
There were some issues having to do with the nginx config that were breaking the renewals, even though the cron job was running (at least, I think that’s what it was; I didn’t pay careful attention to those updates, or I’d have brought this up then)