iCloud says I have over 400K messages stored in the cloud. I've ensured that all devices are only keeping 1 year of messages, but this number seems to keep ballooning and not removing the old messages. How might I fix this without any disruptions to the service?
Well, you posted in an iCloud for iOS region, but your screenshot indicates you’re using a Mac.
You need to view and delete your Recently Deleted messages, but the methods for doing this vary greatly between iOS and macOS devices.
On macOS, you use Messages’ menu to go to View > (something like Recently Deleted) and delete from there. On iOS, you tap “Edit” in the upper left from the Messages app, followed by “Show Recently Deleted” and delete from there.
How to remove excess messages in the cloud? [Over 400K]
