Currently, Meraki does not offer a built-in alert or report specifically for cameras with retention below a threshold like 90 days.
But you can use the Dashboard API to pull retention settings and motion data per camera or create custom reports using the API to flag cameras with estimated retention below your desired threshold.
Take a look at the benefits of Smart Retention.
Smart Retention - Cisco Meraki Documentation
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
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