At a prior job we actually little LED RGB light tabs that would stick on our monitors, and set them to red or green to indicate exactly this. Uptake and responses were somewhat mixed, but it was nice to have an explicit signal, beyond just putting my headphones on, that I was not going to be welcoming of interruptions for the next few hours.

At the time we were in a pretty cramped office in a co-working space, so I appreciated anything that could help cut down on distractions even a little.