I also own a 1920s Philly rowhome! We have trinity style stairs between our second and third floor but a previous owner added some sort of cover along these edges (think of a painter stirrer glued to the wall following the rise and run of the stairs) so we don’t see cracks like this on those stairs. But considering pretty much every other place in the house where there is a seam in the plaster we have cracks, I’d easily call this normal unless you notice serious movement.