I have no idea what they do with long term idle AppleIDs. I don’t work for Apple so cannot say what they do.
But as with any company who you have an online account with, if you cannot prove a login ID and account belongs to you, no company will delete the account based solely on your request that they do so. You have to be able to convince any company the login ID and account are yours, and the standard for doing that is that you can login to your own account.
Apple offers an account recovery process for people to recover their AppleIDs. With iOS 14 you can also choose instead to set a 28-digit recovery key instead. As long as you have or remember that key, you can always recover your AppleID.
But until you recover access to it and control of it, Apple won’t do anything with it since you have no proof it’s your account. If it is your account, you’d either be able to login, or recover it.
You do indeed have the right to delete your AppleID. But you do that by showing you’re able to login to your own account, and request the deletion once logged in. You cannot just ask Apple to delete any old AppleID that you cannot login into or at least successfully recover.