I think it is almost certain that exposure to less-virulent and widely circulating coronaviruses like OC43, BL63 and HKU1 has given many people some degree of protection against COVID-19, and likely explains in part the variability of disease severity.
The open question is to what degree these things are true and to be honest I don't even really know how you'd measure that. I am sure somebody else smarter than me does.