I ran an analysis that compares the date of mask mandates with cases and deaths for coronavirus. No statistical analysis, but the charts and output strongly suggest a correlation
Script output:
cases_total_before 0.16330318584574205
cases_total_after 0.11865204705545934
death_total_before 0.00608007236280604
death_total_after 0.003947986949385576
Charts: