It's an overlay.
2.8 series introduced the concept of Overlays, which are all the things that are drawn on top of your mesh in the 3D view or the UVs in the UV editor.
From the manual:
The Overlays pop-over configures the overlays that are displayed on top of images. In the header, there is a button to turn off all overlays for the UV Editor. This option also toggles the visibility of UDIMs tile information. The options that are visible in the pop-over depend on the UV Editor mode.
Stretch falls into this category.