I had the most success when using the terminology from this article, thanks! "V-notch insert" mostly worked (although I think it's just not a thing people really mention when describing sweaters.)
I really enjoyed reading that page, so thank you for finding and sharing it!
I asked chat gpt and apparently it is also known as a "sweat patch". Its functional purpose is to absorb sweat that accumulates around that area of the chest.
Often see these on waffle knit sweatshirts or heavyweight shirts, high GSM. Heavyweight shirts frequently have unique seams and stitches to keep their shape and strength despite the heavy material.