Why is my old iPhone still receiving notifications?

It seems your old iPhone is still attached to your Apple Account. Do not just wipe that old phone. Assuming you do not want that, please go thru the steps outlined in the article below.

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

If you're dealing with text message forwarding issues or trying to ensure that forwarding is only active on specific devices, you need to:

1. Check all devices signed in to the same Apple ID.
2. This includes iPhones, iPads, and Macs that could be receiving forwarded messages.
3. Manually disable text message forwarding on each device:
4. On your iPhone:
   1. Go to Settings > Messages > Text Message Forwarding.
   2. Review the list of devices and toggle off forwarding for those you don’t want to receive texts. (Note that the listed devices are usually identifiable by name, but you can't see where a specific device is forwarding messages to.)
5. On a Mac:
   1. Open the Messages app.
   2. Go to Messages > Preferences > iMessage and ensure forwarding is appropriately configured.
6. Recheck Apple ID usage: There is no centralized way to see which device forwards messages to which other device, so manually verifying settings on each device is necessary to confirm the forwarding configuration.

If you're not using Messages in iCloud, or messages aren't forwarding as expected:

1. On your iPhone, go to Settings > Apps > Messages.
2. Tap Text Message Forwarding. (If you don't see it, turn iMessage off and back on, then tap Send & Receive and sign in again.)
3. Tap to turn on each device that you want to allow to send and receive messages from this iPhone.
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