The reason messages from iPhones are still going to your old device is that your phone number is still registered with that device's Apple ID. Since iPhone users detect that your number is still linked to an Apple device, their messages are being sent as instead of standard SMS texts.
To remove a phone number from an iPhone and disable its iMessage functionality, you need to follow a few steps, especially if the number is no longer active on the device.
If you still have the iPhone with the number:
Turn off iMessage and FaceTime:
Go to Settings.
Scroll down and tap Messages.
Turn off the iMessage toggle.
Go back to Settings, then tap FaceTime.
Turn off the FaceTime toggle.
Remove the number from your Apple ID:
Go to Settings, then tap your name at the top.
Tap Sign-In & Security.
Tap your phone number to see if there's an option to "Remove from Account."
You may need to add a new trusted number first before you can remove the old one.
If you no longer have the iPhone:
If you've switched to a non-Apple phone and are no longer receiving SMS messages from iPhone users, your number is likely still registered with iMessage. In this case, you'll need to use Apple's self-serve website to deregister it.
Go to the Deregister iMessage page on Apple's website: https://selfsolve.apple.com/deregister-imessage/
Enter the phone number you want to deregister.
Apple will send a confirmation code to that number via text message.
Enter the code on the website to complete the deregistration.