The AttestationServer (github.com/GrapheneOS/Att) alert email system has been overhauled. The alerts still include a list of all problematic (expired) devices. However, it now tracks the last time it sent out an alert for each device and they can't trigger multiple alerts per day.
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This makes the alert emails much less annoying, which was a major complaint about the previous implementation. It still checks for expired devices just as frequently, and will still send out an alert about a newly expired device just as promptly since it tracks this per device.