The files in theme/boost/upgrade.txt and theme/boost/readme_moodle.txt do not describe exactly how or what this version of bootstrap is, and how it was created. Downloading a standard version of bootstrap 4.0.0 and running a diff between this and the one shipped with Moodle produces a random set of changes for things like popover, position, and carousel.

Bootstrap source files should never be changed except in very rare uncontrollable situations, and then they should be clearly documented. We are now faced with a situation where we have an unknown version of bootstrap and have no clear documentation on how we should update to a newer version without introducing changes (4.1.1 for example).