Question: I used iMessage with my cell number in the US. I moved away from the US and cancelled my cell plan. I can still use iMessage to talk to people who have iPhones. But I did not update any information in the app or configuration (such as phone number).

Is there any potential issue with someone being reissued my old number and using iMessage?

Answer: 
There's a potential issue for **any** app or service you have that uses your phone number as a primary or secondary address source. You need to remove/change the number in everything from iMessage to WhatsApp, and also any service you use which has your phone number as a 2FA/backup/lost password path.

The time from abandoning an old number to it being re-assigned to a new user is provider-dependant, from 90 days to a year.
When you de-activate your phone number it should de-activate from iMessage. Your messages will start coming from your email address, and people will need to send new messages to your email address.

Your existing messages will not be deleted, and if you switch to a new number or an email address, you will continue being able to send/receive messages.