You think the only thing that happened in the last year was masks?
Just straight "kids mingling at school" is such a major spreader of the flu that long before Covid came there were studies comparing flu rates before and during spring break and showing big drops. Now imagine you don't just cancel school for a week, you cancel it for a year. And on top of that you tell the kids not to mingle too much in other settings either, at least not indoors. Then on top of that half the adults were working from home. On top of that you aren't having concerts/athletic events other big crowds. On top of that you don't have people hanging out together indoors for extended periods of time, and when they are together they wear masks.
The # of separate factors that have led to the decrease in flu rates is immense.
I don't know how they do it in other countries, but in the USA the flu rates aren't measured by people "blaming" their case on the flu, they're measured by routine sampling and direct lab tests. They report the results of these every week.
For example, in the most recent week for which results are available (week 16 of this year), the CDC tested 19,651 people with flu-like symptoms and only 18 tested positive for the flu (obviously the vast majority of those were covid, but sometimes people get other viruses like bad colds or other coronaviruses). Since this year's flu season started on September 27, 2020, the CDC has tested 959,337 samples and only 1,792 have come back positive for influenza.
Weekly U.S. Influenza Surveillance Report | CDC