An actual manhole cover, this is either going to transcend shitpost or literally define it.
Nobody in their right mind would put that inside, so was either built by someone not in the right mind or an extension that just paved right over it.
Manhole covers are used for structural rigidity and tamper prevention.
First (and most hopeful) thing that would come to mind is a rainwater sewer.
No idea of country of origin, it could be someone's idea of a protective measure for a boundary trap If the sewer main runs directly under the property which would be a bit unusual.
If it is a shaft to a regular sewer, I think you'd smell that.
Rent's going to go crazy if it turns out to be a parallel dimension attached to a mansion or something.
Of course, sir, I love to have my taps split into warm and cold and having my shower head bolted to the wall at a height no man could possibly be comfortable with.
IIRC this was a reddit post, but without the Snapchat text. I do have a vague memory of the OP being from Britain and that they came to the conclusion that it was some sort of sediment tank.
Elevating the most realistic comment from the linked thread:
This is quite common for extensions in the UK at least. If there is a man hole you can often get permission to build over it but you would need to be able to provide access should it be needed. In the 15 years of living in a houses with one under our kitchen no one has ever needed access. These is one just outside in the side passage so likely most blockages should they arise could be cleared from that one. A previous property was similar but they had tiled and concreted over so would make a mess should they ever need access.
Edit: one in our kitchen, if access was needed they would need to move kitchen units and break grout. https://imgur.com/gallery/OU0jFvk