My professional advice is to knock it over with a lawnmower, Then let the grass grow up around it for a few weeks until you forget it's there. Shortly thereafter, accidentally run over the pipe with the lawnmower in order to destroy at least one blade. When the lawnmower is finally fixed after several months, go mow the lawn again, which has grown to a ridiculous hight at this point. You can then forget the pipe ever existed, it's gone forever.

I've done this with several large metal objects, works well 😅

I think it's supposed to be a vent for the leech field. See https://septicpatrol.com/does-a-leach-field-need-to-be-vented/

I'd take a trip to the local septic permitting agency and see if I could get copies of the permit and approved septic system design/as-built site plans. The inspector will probably be able to tell you if you need it or not.

It might be a vent for the leach field although that's not common where I'm from. I guess it could also be a marker for a distribution box or some other system component.

technically it's your lawn, you can do whatever you'd like to it.

the real question is should you

It’s not attached to anything and I can pretty easily pull it out and put it back in, although I’ve tended not to touch it much.

Back in to what? Does it attach to something or did you literally just pull it out of the ground?

Just rejoice in your free pre-installed bird-feeder holder!

How far into the earth is it going?

If you're pulling it out and putting it back in any appreciable distance, it's probably already plugged. Also if it comes out without unscrewing it might just be a marker. What does the in ground end of the pipe look like? Is it threaded? It could be someone is just having a laugh...

Do you contract for maintenance? Talk to them.

If you don’t, and someone has been out, maybe you got their name, or company?

Reaching out to someone who’s worked on the system could save you a bundle in the long run.

Whatever you do, don't get the city involved. That won't end well.