Not sure how accurate this site is, but it seems to make sense to me
Changing, merging, moving, or deleting iCloud email addresses is tricky, because Apple doesn’t allow to merge accounts. These are your options.
michaelnordmeyer.com
Deleting an iCloud Email Address
You cannot delete an iCloud email address without deleting the Apple account it belongs to, because this email address is the Apple ID for this account.
There may be very old accounts (mac.com or me.com), where your Apple ID is your self-chosen username. Then you might have a chance. But if you change the username, you have to use an @iCloud.com email address for the new user name.
Deleting an iCloud email account wasn’t possible in the past, but has been
made available by Apple in May of 2018 to support the
European General Data Protection Regulation (GDPR). This is mandatory for users in Europe, but Apple thankfully implemented those features for users world-wide.
Deleting an iCloud account would mean a user cannot login anymore and purchased items like movies, TV shows, eBooks and apps for iOS and macOS wouldn’t be accessible anymore as well.
A deleted iCloud email address cannot be reused by anyone, even by yourself.
More information from Apple Site
As part of putting you in control of your data and privacy, Apple gives you the ability to permanently delete your Apple Account at any time and for any reason.
support.apple.com