I could have misinterpreted the up-thread comment, but I read it as they were saying that the immune system can degrade its available response to certain pathogens when the immune system goes without adequate exposure to those pathogens.
The wearing of the mask is just one piece in this puzzle. Hygiene and sanitary practices vary widely across Asia. It could be that wearing a mask degrades maintenance of the immune system, but other sanitary practices in Asia result in the opportunity for the immune system to maintain vigilance. That is, it could be the immune system maintains itself in spite of the masks, rather than the mask having no effect or positive effect on immune system.
This is but one source to support that people in Asia wear masks at a high rate. [1] [2]
[1] https://www.ncbi.nlm.nih.gov/labs/pmc/articles/PMC3536629/ [2] https://www.psychologytoday.com/us/blog/culture-shocked/2020...