I have a closet between my living room and kitchen. Any reason that you see why I couldn't take this down? Thanks for any help.

We live in a 2-story Georgian (w/ basement) built in the early 1950s. We recently discovered a busted drain pipe running thru an interior soffit in our Kitchen. The plumber suggested it was a longstanding problem (many years) even though we just started noticing any evidence of the problem (water leaking into the kitchen thru the wall/soffit). When the plumber opened up the soffit to replace the pipe we noticed that the underside of the floor above the kitchen and the floor joists were jet black - almost if they had been sprayed with something. At first, from a distance, we also thought there was a significant amount of black mold, however, upon looking closer, it looked more like the creosote buildup you might find in a chimney. In some sections, it looked almost like it was 1/2 inch to 1 inch thick. The plumber thought it might be insulation sprayed with some sort of black tar. Any thoughts on what it could be? Harmful? (I apologize for not having photos to share at the moment)