With respect to instances that are terminated in AWS console , we suspend it immediately for licensing purposes.
EC2 instance will continue in suspended state for 10 days in Site24x7. Post that , we remove it on our side with a clean up process.
So , post 10 days , the monitor will not be visible in Site24x7 console.
Hope that clarifies your question.
Kindly let us know if you have any requirements with respect to clean up.