Which of the following was NOT introduced in WCAG 2.1?

The inclusion of 1.3.4 Orientation, 1.4.10 Reflow, and 2.1.4 Character Key Shortcuts in WCAG 2.1 signifies enhancements made to address emerging accessibility needs, especially for mobile and touch devices.

1.3.4 Orientation ensures that content remains usable when the device orientation changes, accommodating various users' preferences. 1.4.10 Reflow improves presentation by allowing a page to be viewed in a single column without losing content, which is important for users with visual impairments who may need to zoom in. 2.1.4 Character Key Shortcuts introduces new specifications for shortcut keys, helping to address keyboard accessibility for users with mobility impairments.

On the other hand, 3.1.2 Language of Parts is not a new guideline introduced in WCAG 2.1 but was part of the existing WCAG 2.0 guidelines. This success criterion focuses on providing the language of specific parts of the content, allowing screen readers and other tools to assist users better in understanding the content. Therefore, it was not a new introduction in WCAG 2.1, confirming that this choice is the correct answer to the question.