The masks indoors thing doesn't apply well to BGW anyway. Other than the recently brought back indoor shows at theaters, almost every indoor space at BGW is consistently open to outdoor airflow on a scale significant enough to keep the air inside the buildings relatively refreshed. Festhaus is probably the only exception to this I can think of because of the sheer size of the indoor space, but even then, 90% of the individuals in the building are sitting and eating, an activity that you can't mask up for anyway.
I appreciate the gesture, but I see this as more as a thing to look like they care, since the actual effectiveness is extremely trivial. Perhaps the one area this has merit is the Abbey Stone/Globe which would make sense if the sign was there, but park-wide it seems more like a PR move.