Corrective Shape Keys is a Blender add-on that creates a corrective shape key for the current pose. It was part of Blender 4.1's bundled add-ons and is now offered as a standalone extension, provided as-is with limited support.
The add-on lives under Object Data → Shape Keys Specials and can also be reached through Blender's Search. It's compatible with Blender 4.2 LTS and newer, requires no special permissions, and comes as a small 4.7 KB download. The original authors are Ivo Grigull (loolarge) and Tal Trachtman.
Released under the GNU General Public License v2.0 or later, the extension carries version 1.0.0 and has been downloaded more than 24,000 times from the Blender extensions platform. There's no changelog for the current release.