Not sure what would lead anyone to say it was a lack of maintenance from the pics above. They look as if it was as you stated, a sheared portion and then it appears to be just a broken away area. I think it was the cage catching in the spokes (what reason this happened I couldn't say,) and it forced the derailleur onto its stop and broke the piece off. This is my guess, a speculation only.

Unless you think that something you did, or didn't do, caused the cage to go into the spokes, it seems that something forced the cage into the spokes. What caused this is the mystery, the plot thickens.

Bill