Spam filters will get you cleaner data moving forward, but they won’t be applied to past data.
Once you’ve set up your filters, spam will be filtered out from that date onwards, but not backward. Your historical data will still include inaccurate data caused by bots.
However, it’s not all bad. You will be able to compare your with and without spam data in Google Analytics.
Seeing the comparison between your analytics before and after applying spam filters can help you understand just how much traffic was from bots, giving you a more accurate view of your site.