“Device Added to Your Account – A Mac now has access to iMessage…”
I don’t see any unrecognizable device in this Apple ID account in this 2020 13" Intel MBP’s macOS Ventura v13.6.9. Here’s a cropped screen shot of its notification:
Thank you for reading and hopefully answering soon. :)
I occasionally see this. Typically after one of my devices goes through a system upgrade. I suspect the upgraded macOS/iOS needs to get new authentication keys/tokens from iCloud, and when it gets them, your other devices are notified.
You can log on to Apple to review the list of devices on your account: https://appleid.apple.com/account/manage/section/devices
If you don’t see anything unusual there, I wouldn’t worry about the notification.
I would love to see some way to get a list of devices signed in to your iMessage account (which is not necessarily the same as those signed in to iCloud), but I don’t know of a way to get that list.
Apple needs to revise that message since it is confusing and freaks me out. Your idea is good! Please leave a feedback to Apple – Product Feedback - Apple. I just did with a link to my thread. ;)