What are these capped, metal pipes in our yard?
Even though 811 ignored them doesn't mean they are not a private utility. It is possible they are natural gas or waterlines, or were an electrical conduit however they are most likely just pipes driven into the ground as support for a past fence.
To rule out private utilities you can call an HVAC contractor licensed in PA to work on gas (most installers and maintenance companies have employees who are) to come out and verify it. They will likely uncap it and use a special meter to detect gas. If there is water or electrical lines they will be able to tell you and give you guidance on your next steps. They will need access to the home and it will be easier for them and you if you clean out any areas in the home where pipes or electrical lines could be concealed behind exterior walls or crawl spaces on the side of the property the pipes are on.
Absent any utilities, you can dig down to the base or use a pipewrench on the pipe and unscrew the pipe and pull the unscrewed pipe up. If it is welded to a spade or something weird, you'll have to dig down all the way or cut the pipe off below grade.
If you have never worked with natural gas I don't think it would be responsible for you to take any advice walking you through dealing with a suspected gas line other than to call a licensed and trained professional.