@Jeff Smith Interior wall is standard 2"x4" framing, exterior is brick. In between is just standard insulation, lots of it.

The last people who remodeled did it this way to avoid having an awkward eave on the largest wall in the unit, but I just hate knowing there's usable space back there going to waste.

Well, creating storage niches is simple, then.

Plan how big (wide and tall you want the niches. Presumably they will be less than about 14.5" wide (16" minus 3/4" times 2), otherwise you'll have to remove 2' x 4's. If it's a non-loading bearing outside wall you can remove studs; but check with an engineer or carpenter. It's likely load bearing if on outside wall. With the age of the house your studs may be farther or closer apart, too.

You may also want to pack around the inside of the box (niche) with hard styrofoam insulation, which has greater R value than fiberglass. May also want to use canned spray foam to further seal? This way after removing some insulation you won't have as much of a cold spot on outside wall where niche is.

There's a number of pre-made (pop-in) solutions like this:

https://www.wayfair.com/storage-organization/pdp/wg-wood-products-bal-harbour-155-w-x-495-h-recessed-shelving-wgwp1493.html

This example is interior wall. See here for a DIY:

https://www.diynetwork.com/how-to/rooms-and-spaces/walls-and-ceilings/how-to-make-a-wall-niche

Second approach, similar to above but narrower:

https://www.familyhandyman.com/storage-organization/build-a-wall-niche/

The original builder didn't build the wall that thick for no reason. Drill a small hole (say under one of your framed artworks) through the surface material of the exterior wall and stick a coat hanger in to see what you're dealing with. My guess is not much.

Are there any windows in the wall -- such as dormers? If so, can you get to the side of a section of a wall?

You're going to need more of a look than thru a pinhole. I suspect you may find insulation, etc., in the wall. The only way you're going to know is to pick a section of wall and remove the interior drywall or paneling ... very carefully so you don't damage any wiring or plumbing or duct that might be in the wall. Post before and after pics so the Houzz pros can see what's there and give you some guidance.

You might look up under stair storage for ideas and consider drawers -- including vertical drawers -- in addition to traditional "closet" storage.

@seabornman The original builders didn’t finish the space, it was an attic back in the late 1800’s and early 1900’s. The wall was added at some point after that when this was converted into a three family

This wall is the only wall with no doors on it in the whole unit, it’s the only logical place for a couch and table, so I’ve always assumed that whoever built it just dropped it down from the standard ceiling height to maximize the livability of the space.

I know there’s nothing in the wall beyond insulation and framing. My question is more how to go about this and what problems I may encounter as i can’t find any good examples of this type of thing online. I know there’s at least some useable space back there.

If the wall was added, it's probably not structural and you can do what you want, regardless of stud locations. The only way to know is to follow Suezbell's suggestion and cut a hole big enough to see what's going on, preferably near the top, so you can see the framing above. You'll have some drywall work to do anyway if you want to do this.