Depreciated in 10.3?

Agreed. Same faulty info @ https://www.drupal.org/docs/core-modules-and-themes/deprecated-and-obsol... →

Seeing the same deprecated info here as well

You don't need to add this project to your codebase. You just need to install drupal/core and enable the sdc module that is included there. That's all. It's just about where the code lives (and is developed further). This namespace only if you will is deprecated.