Deleting stale devices causes lots of device updates to be sent out over federation

[matrixbot]

This issue has been migrated from #14651.

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matrix-org/synapse#14595 introduced a background job code that runs in DeviceWorkerHandler.check_device_registered to remove old, non-e2ee devices for users that had >50 devices, in an attempt to reduce the number of devices on homeservers.

When this was applied to matrix.org, it ended up spamming homeservers over federation with device list updates.

We may want to consider a rate-limit for the amount of devices we remove in this - and future - cleanup jobs.