Or maybe it's just been misplaced? I noticed it was missing from the Forest Service and National Weather Service maps. In actuality, it's still physically there as of a couple of days ago. 
I swam in it and walked the trail around it. There was a Forest Service trail crew camped out and doing clearing of logs and boardwalk replacement so the trail is in good shape.
My brother has a new truck with a fancy navigation map and Monon Lake is also missing from that database.
You can still kind of see the outline of the lake on these maps, but they've masked it for some reason. I wonder what's going on? 
That is very weird. The maps I am using for the map page here, the Thunderforest and Mapbox maps both show it but the ESRI New Topo does not. I'm guessing ESRI is probably the source of the missing data. But I can't come up with any good reason why?
Very very weird.....
From the ESRI website:
"Today, Esri software is deployed in more than 350,000 organizations, including the world’s largest cities and most national governments."
If the Forest Service and National Weather Service are clients of ESRI, then maybe some bureaucrat decided Monon Lake shouldn't show up on the public facing maps. It makes me wonder what the rationale is.
The trail page is still active for the Monon Lake Trail and Monon Lake still shows up on the map at the USGS website.