We did a bathroom renovation on a house built in 1912. During demolition we unearthed thousands of blades between the two studs to the right and left of the sink. My hypothesis is there was a slot in one of the previous medicine cabinets. I couldn’t find any signs of a slot in the plaster and lath. A bit of a digression, but thought you might find it interesting.
I have heard of this as well. And apparently it is still a phenomenon: https://www.rd.com/article/if-you-live-in-an-old-house-there-could-be-razor-blades-in-your-walls/