Security reasons must have become a secondary concern now that the presence of COVID-19 could be just around any corner or right in the person in front of you. Wearing face masks may not be required from everyone in casinos but I think they need to start observing healthy precautions, especially in tourism casinos where the majority of the clients are coming from everywhere in the world. The risk is too high for these casino workers. Prevention is always better than cure.
Casino players here in my place are not prohibited nor required to wear face masks but the standard protocol has always been observed at the entrance; everyone will have to remove their face masks, caps, shades, and whatever headwear they're wearing for identification and recording purposes.
It's funny they say you have to remove head-ware (baseball caps etc) and then you wander over to the Texas Hold Em tables and the sharks are there with the dark glasses (even at night indoors) and caps or even the classic poker cap trying to hide their gaze from others.
I do not know the process in other countries but in most establishments here in my particular city, not just in casinos but also in schools, malls, and so on, anything worn in the head or in the face such as face masks will have to be removed as soon as you enter, but when you are already inside you are free to use them. Those are not prohibited, after all. You just have to remove them in the entrance so that the security personnel and perhaps the CCTV as well could have a clear view of your face. That is basically a security protocol. When you are already in the Holdem table or just wandering around jumping from one game to another, you are completely free to wear all kinds of masks and shades and caps.