It's an extremely dirty picture of the die. I think they chemically decapped a previously mounted die that was used for failure analysis rather than take a picture of a pre-assembled die. Unlike the German die photographer they didn't spend hours to make it piece of art.
In other words: the die had been subjected to a lot of abuse before they took the picture. Those diagonal cuts could be anything, but they're definitely not something you'd do to disable a functional block. That's something you want to do after the die is mounted on the interposer and mounted on the substrate: all those steps can result in silicon failure. You want to disable stuff as late as practical.
Maybe they didn't want to use a clean picture to prevent detailed competitive analysis.
That makes no sense. Cut or no cut, deliberate or accidental, it's an extremely precise feature - it goes from the precise corner of one block to the diagonally opposite corner. Of a block that is repeated many times across the rest of the die, so what features could be hidden from competitive analysis by doing that to that one block?
Anyway, I'll bow to superior knowledge. Perhaps it's just a scratch made by a woodscrew.