Is this actually still connected and used as a clean-out for your drain pipes?

You could probably dig down around it to get access, and find the point where the outside is just a standard round pipe. Then use an angle grinder to cut it off evenly, and use a Fernco coupler (with the outside metal sleeve) to connect this pipe to a new PVC pipe. Fast forward to about the 1:00 minute point in this video to see how the coupling works. https://www.finehomebuilding.com/2018/03/14/tie-cast-iron-pipe-pvc

Then glue a fitting on the PVC that will allow you to screw on a standard pipe cap.

Bruce

Are you SURE that's a clean out? Are you on city water? That looks like an old curb key that's missing its cap to me. Does it line up with your water supply?

Interesting thought, yes I am on city water. I was assuming it was a clean out due to the location in the yard.

What are my options if it is a curb key? Go through the water company?

It is a curb key. Yes, talk to your water supplier. They will at least provide a cap.

Good save, Jake!

looks like