I wish people did this. No, I'm not saying it should be mandated, but I guess I'm saying it shouldn't have to be mandated. If you (royal you) think you might have a contagious disease (cold/flu/etc.) please limit your interactions and wear a mask. If you can, maybe also let others know so they can choose to further distance and/or wear a mask. I'm not saying people need to shut themselves up in a bubble if they get sick, but it doesn't seem that hard to have a little common courtesy if you think you could be contagious.
We had an in-person department retreat last week, and two people we wearing masks because they had colds/sore throats/etc. I appreciated that, and knowing that information I also choose to wear a mask because I had some travel coming up and wanted to avoid getting sick (regardless of whether it was COVID/flu/cold/etc.)