Based upon its location, I would assume an overflow port.
When it rains hard does your pool water stay below the bottom of the coping?
You can stick a garden hose in it and check and see where the water comes out. Most likely tied into the deck drains, so you could listen and see if you can hear the water running in the drains.
Given the fact that water is half up the opening, it’s either got a shutoff valve somewhere near the pool in ground (look for a plastic landscape box) or whatever it is tied to is completely plugged up.
Thanks guys, I may try running a hose through it tomorrow. I added an inch or two of water and tried turning the pump on, but couldn't find any water exiting the waste line or anywhere within sight.
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