Location:

Face menu - Inset Preserve Corners (at the bottom)

or

RMB (right mouse button click) - Inset Preserve Corners (at the bottom)

Usage:

Essentially this is the same inset tool but with a feature to preserve corner edges.

• Cut-off Length - maximum length for edges in corners to preserve their deform during inset operation. It automatically calculates the shortest edge length (with a gap of 1.5 lengths), you can adjust it in the undo panel.