Not a problem.

To plug the hole, id just glue a little bit of wood shaped to fit the hole in there.

Given the angle of the wood to the wall in your pics, it almost seems that somebody finished the subfloor, which is installed on a bias to the final flooring. No matter. Forensics can be tricky in old houses, which is all I’ve ever lived in. It’s also hard to judge the diameter of the hole from your pics. Diameter of the hole is the critical issue, with final finish a close second, and function kind of overarching everything. Assuming your right, and it’s an old, vestigial 1/4” gas line for a floor heater or radiator, then the pipe must be either sealed on this end, or disconnected at the other. If that’s the case (and in most cases) a small shot of aerosol foam sealant (Great Stuff) is the ticket. Let it dry 24h, slice the top off with a serated kitchen knife, and then dab it with a little brown paint to match (‘ish) the floor. It’ll be sound/air proof, visible for future renovators, and quickly disappear into the background visually. Smaller holes have other solutions, and shoe molding can be lovely. But, cheap and simple is always best. I love those old buildings by the way. Twenty coats of paint on the baseboards, and still counting. NOTE: Coaxial cable was often run through the floors like that, so if that’s what’s in the hole, or the hole is smaller, then use the same solution or just successive coats of painter’s caulk to fill the hole. 🚀

Thank you! Upon further inspection the pipe seems to be full of drywall anyway

I used wine corks on my floor holes and they worked great.

Thank you so much for this comment. The diameter of the hole is just under an inch and a quarter. Upon closer inspection, and after scraping away a bit of the sealant with a tweezer, it seems that the pipe has a fair bit of drywall or something at the end of it blocking it off like you say. This makes me not worried I've created some sort of fire hazard or anything. I think you're right about it being subfloor and that is very impressive you could tell that from a pic. The floor in other areas of the apartment appears newer, is angled differently, and seems to sit just above the level of the floor in this room (not by much just a quarter inch)

If you want it to be easily removable, use silicone sealant around a piece of wood or cork.

My pleasure to help, and I sincerely hope all turns out well. In terms of it being hazardous, there’s little reason to suspect that’s the case if this hole/pipe has been there all along. You might wanna check for carbon monoxide, which is something you always wanna keep an eye on. Presumably you don’t smell anything. But old fittings are very common. They’re rarely removed by DIY homeowners or contractors, mainly to keep costs down. For fun, you might wanna do a little snooping in a crawl space or attic, if there’s any such access available. Start with your gas line theory, which sounds reasonable. The pipe should be tarnished copper, or maybe GSP (galvanized steel pipe). If the building was fitted with gas, you should see gas lines of identical material in other utility areas, like the laundry room, kitchen, bathroom, hot water heater and HVAC. Look under sinks and inside closets. Gas lines will typically follow plumbing lines due the “chases” required, which are the accommodating spaces built into the architecture, but not always. Ask an older neighbor who owns a house/unit built at about the same time as yours. You’re first instinct - to be cautious- was your best. It’d be best to identify it, if practicable. Again, the forensics can be fun, and you’ll likely develop useful information when other projects come up. 🚀

judging by the irregular shape of the hole, that appears to be a knot hole in the wood.

as others have said it would be best to plug the hole with something like a wine cork or wood dowel, then use sealant.

I’m a city inspector and I get complaints like these all the time….this is a trip hazard and needs to be remedied asap. Call your city and ask for the rental dept.