Adsense will stop allowing publishers to see their data older then 3 years
Google currently displays this info in the Adsense backend:
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Last 3 years of reporting data
We've limited reporting data for all AdSense products (Adsense for content, AdSense for search, etc.) to the last 3 years. This means that you can only run reports for data that's less than 3 years old.
You can download data that's older than 3 years from either the old Reports page (until the end of September) or the new Reports page (until the end of 2020).
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I wonder why they do this. They don't say they will delete the data. Just that they will stop giving publishers access to it. It will be harder to see larger trends and repot on them. Avoids bad publicity when something is up 100% or more over 10 years