delete alias

See Use email aliases on iCloud.com https://support.apple.com/en-nz/guide/icloud-ipad/mm6b1a490a/icloud

Now see iCloud: About your @icloud.com, @me.com, and @mac.com email addresses https://support.apple.com/en-us/HT201771.

If you created one or more email aliases for your account, be aware of the following:

• You can change or delete an email alias, but if you delete an @mac.com or @me.com email alias, you can't add it back to your iCloud account.
• You can't sign in to iCloud.com with an email alias. Sign in with your @icloud.com email address or the Apple ID that you use with your iCloud account.

I have always thought that you could not delete an @mac.com or @me.com email alias, but this apparently that may not be true. But, perhaps this document has not been updated as appropriate.

Note that it mentions email aliases and their deletion. See Use email aliases on iCloud.comhttps://support.apple.com/en-nz/guide/icloud-ipad/mm6b1a490a/icloud

If you mean you have another version of your email address - for example an @icloud.com one and also an @me.com one, with the same text before the '@', as a result of your having been a member of Mobileme, they are in effect the same address and you can't delete either version.