The problem

Apple device management sucks. Apple's profile management across various parts of their services and servers also sucks. Music settings do not apply to Apple ID and vice-versa. Device authorizations in Apple ID are not necessarily the same as Music authorizations. And so on. It's frustrating and unessesarily complex and stupid.

Simply deleting the device from your Apple ID is not good enough. Oh-no, you have to go through a multi-step tedious process of fact-finding and frustrations to figure this one out. I've spent hours pulling out what little hair I still have on my head, following guides from Apple that casually explain how this works as if any sane person would ever think of this. The pages I followed either send me to non-existing menus, to settings that didn't apply to me or my account or it simply didn't do what Apple promises.

Terrible!

Having settings scattered over every device, app and their website is no help either. Double terrible! But I think I figured it out.

Removing an old Apple device from your account

on macOS

In System Settings, scroll down to 'Media & Purchases', click it and click the 'Sign out' button on the right. I think that this de-authorizes the device from buying more stuff.

But wait... There's more!

If you've ever bought anything with this device in the App store or Music store that needs to go away too. Severing ties to that purchase so-to-speak.

In Apple Music (the app), in the top menu click on 'Account', in it select 'Account Settings'.
Sign in when prompted and in the pop-up window scroll down to 'Devices' and remove your device there too.

But wait... There's still more!

Similarly, you need to sign out from the App Store as well. Open the app and in the top menu click 'Store', in it click on 'Sign Out'.

Now go back to System Settings and sign out of your Apple ID. And that should do it!

On iOS/iPadOS

This is a tiny bit less frustrating, but equally dumb. You can just reset the device like I did and think it'll be done. But that's not how Apple works.

First, connect the device with a USB cable to your computer. In macOS Finder open the device and disable backups, delete all old backups too. Yes, skip the current backup being made. I had to click like 20 times for it to finally listen. Finder is stupidly insisting on making a backup.

Once it finally stops being a nuisance you can finally remove all previous backups, until it says something like "Last backup: Never".

Then uncheck 'Show this iPad/iPhone when on Wifi' and 'Automatically sync when this iPad/iPhone is connected'. Also uncheck 'Manually manage Music, movies and TV Shows'.

Next click Apply or Sync in the lower right of the window and wait for the device to do its thing.

NOW you can reset the device as you normally would. Open the Settings App and scroll all the way down. Tap on 'Transfer or Reset iPhone' and follow the steps in there.

But what if you don't?

I didn't do any of the above when I got my new iPad earlier this month. For over a week I've been staring at Finder wondering why my iPad showed up twice. Of-course because I'm oh-so creative with naming devices and I gave the new one the same network name. But also because Apple, even after factory resetting the device, insists it's mine and I should do stuff with it.

Today Music tried to sync the old iPad and kept stalling the app, because it couldn't. It locked up the whole thing. And why would it work, or even think it's a possibility? The iPad is reset and doesn't know my computer, or what my Apple ID is. Yet macOS and by extension Finder and Music kept insisting it's still an active part of my live.

Until I did the iPad removal steps.

The new device

As I mentioned, I bought a new iPad to replace my old one. The new one, while associated with my Apple ID, showed up almost nowhere else, even thought it's logged in into my Apple ID and set up with Apple Pay. It's not authorized for Music, yet I can sync my playlist and play bought music on it without issue.

I have made purchases in the App Store. So what all this signing out nonsense is for...?

No idea! But the old one is finally gone from my Macbook Pro!