What is the «Tinymce3» (`Magento_Tinymce3`) module for?

This module provides backwards compatibility for TinyMCE3 for clients that have modified the TinyMCE3 editor in Magento and don’t want to lose functionality.

github.com/magento/magento2/blob/bce891d2/app/code/Magento/Tinymce3/README.md