The retention period for flow run history is defined by the Flow Run Retention Policy in Prefect Cloud, which is based on your account tier. This policy dictates the number of days a flow run is retained in the UI, CLI, and API after it reaches a terminal state. Note that if you can't open older runs, they might have been removed according to this policy.
If you're experiencing trouble viewing old Prefect flow runs, it may not just be because of retention policy limits. It could be an issue with Prefect UI or API. There were enhancements in the past where the ability to traverse between flow runs and subflow runs was improved, which could affect how you view them now.
If your issue persists, you might need to search GitHub for related issues or reach out to Prefect support for assistance with older flow runs that should still be within the retention period.