What is this old outlet with the weird slot shape?

That right socket is almost certainly a radio antenna outlet. Houses built in the mid 20s through the 40s sometimes had dedicated wiring for a roof or attic antenna, and the outlet took a special plug that fed the lead-in wire to a radio set inside. The odd slot pattern, one horizontal and one angled, was a proprietary arrangement so you couldn't plug an appliance into it and vice versa. Nothing household goes in there and nothing modern is made for it. It's a relic, safe to leave dead. I ran into the same thing in my aunt's 1930s cape. There was a similar surface receptacle behind a bookcase, and when we traced the wiring it ran up through a closet and out to a gutter-mounted antenna that had long since rusted away. The feed ussually lands on its own seprate ground rod outside too, driven just for the radio circuit, not tied back to the panel like a real equipment ground. So the left socket may or may not be grounded even if the box itself has a wire on it. For the left side, a plug-in tester with the little light pattern will tell you if it's hot and wired normally, but on a knob and tube era job that only goes so far. Old surface wiring on wooden panels often has cloth insulation that gets brittle and crumbles when you look at it wrong. If the tester reads right and the plug sits snug without arcing or warmth, a lamp is fine. I'd have whoever checks it also confirm the circuit isn't still fused off some ancient panel somewhere.

The antenna wiring itself can just be abandoned in place if it's dead, which is what most people do. Cutting it out is more trouble than it's worth unless you're opening that wall anyway.

Ha, those radio outlets turn up every so often if you've been crawling around old houses long enough. Found one behind a dresser in a 1928 farmhouse years back, and the original antenna plug was still sitting in the drawer with the radio manuals. The fancy faceplates were a selling point back then, dealers wanted them looking nice on the wall. Sometimes you can still spot the flat lead-in wire running up under the eaves outside, or little porcelain standoff insulators along the roof edge. If you see those, you can trace where the antenna used to hang. I'd leave the whole thing exactly as is, dead or not. Nothing wrong with a little history on the wall.

I'd pump the brakes on calling it 'almost certainly' a radio outlet before anyone's actually traced that wire. I said the same thing about a wierd receptacle in my old rental, got halfway through assuming it was antenna stuff, and it turned out to be wired off a lighting circuit that was still live. In my case it fed an old doorbell transformer setup the landlord never knew about. Point being, those odd slots do match the radio plugs I've seen photos of, but I'd pop the cover with the breaker off and look at what's actually connected before deciding it's dead. Cheap to check, and it settles the question in five minutes.

Update: found a faint stamped label on the faceplate under the dust, looks like it says antenna, so that theory holds up. Still stuck on the other part though, wheather the wire is actually connected to anything. How whould I tell if the lead-in is still run up to the roof, or do I need someone to trace it from the attic end?