"proxy for how serious the wearer takes Covid as a threat." I would suggest that this now law in some countries and that would be the reason for wearing facemasks.
The true covid death rate has been tricky to record in the UK, a positive test result within 28 days prior to death is recorded as a covid death, even if the death was caused by something else. We obviously don't know enough about COVID to know if it is contributing all the COVID registered deaths.