All guess work:
That wall is an edition. If it was original to the house as a stiffener, it would be sitting directly on the lath.
The little 'blue' pieces raise the 2x4 above the plaster lobes. If scraped completely flat, the ceiling below would have got all messed up. Even then, it's likely to pop the plaster off the lath, with what little is left sticking up.
I hope there's a top sill inside the room, if not then it's counting on the lath to not fall over and counting on itself to not let the wall rack. (Drill a hole through the 2x4, and then keep going. If you hit another 2x4, then at least you know the wall won't rack but it's still counting on that lath for stability. Install braces throughout.)
What I would do anyway:
Cut out the section you need removed and then install ledger boards perpendicular to it and its adjacent joists, on both ends of the cut. (If we're counting on some adhoc inner wall to not let the house rack, the house is fail.)
Old construction will take a surprising amount of hackery before it becomes unstable. Just don't ever notch a floor joist or modify a support post and you'd have my blessing.