Retrieving CPUUtilization for terminated instances

CloudWatch Metrics retain all running/terminated instances metrics, here is the timeline

What is the retention period of all metrics?

CloudWatch launched High Resolution Custom Metrics on July 26, 2017. This enables you to publish and store custom metrics down to 1-second resolution. Extended retention of metrics was launched on November 1, 2016, and enabled storage of all metrics for customers from the previous 14 days to 15 months. CloudWatch retains metric data as follows:

Data points with a period of less than 60 seconds are available for 3 hours. These data points are high-resolution custom metrics.
Data points with a period of 60 seconds (1 minute) are available for 15 days
Data points with a period of 300 seconds (5 minute) are available for 63 days 
Data points with a period of 3600 seconds (1 hour) are available for 455 days (15 months)

Data points that are initially published with a shorter period are aggregated together for long-term storage. For example, if you collect data using a period of 1 minute, the data remains available for 15 days with 1-minute resolution. After 15 days this data is still available, but is aggregated and is retrievable only with a resolution of 5 minutes. After 63 days, the data is further aggregated and is available with a resolution of 1 hour. If you need availability of metrics longer than these periods, you can use the GetMetricStatistics API to retrieve the datapoints for offline or different storage.

https://aws.amazon.com/blogs/aws/amazon-cloudwatch-update-extended-metrics-retention-user-interface-update/