Do you use Grammarly? Grammarly adds this tag automatically.
It looks like it is the Grammarly 79 extension that adds that data attribute 34. Likely use for some internal check (like to verify it has loaded into the page).
See https://forum.freecodecamp.org/t/what-is-data-gr-c-s-loaded-true-attribute/373769
You can also check with if (document.body.dataset.newGrCSLoaded), which is the newest data attr for Grammarly if the user has Grammarly installed.