As a seasoned design professional I have memorised most of my shapes....triangles, circles... even rectangles, but I need to do some research on this shape, and for the life of me I cannot think "what to google"! "Square with two rounded corners" isn't working!
Microsoft calls it a Round diagonal corner rectangle. Here are the names of various rectangles (one can confirm this for oneself by mouse-hovering over these shapes in e.g. Powerpoint or Word):
Given a squircle is a square with rounded corners, you could call this a "half-squircle"*.
* Technically the shape you mention does not fit the mathematical definition a squircle, but it's close enough to be called that by most people in the same way that people call things that are approximately circular a circle.