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.