Filters may be applied retroactively to any data that has already been processed.

Filters in Google Analytics are applied to data as it is being processed and stored. They are not retroactive, meaning they cannot be applied to historical data that has already been collected and processed. Once data passes through a filter, the filtered version is what is permanently stored in the view. This is why it's crucial to test filters thoroughly on a test view before applying them to your main reporting views. If you apply a filter incorrectly, the historical data in that view will remain unfiltered or incorrectly filtered, and you cannot undo the effect on past data.