We are using an older version of Bootstrap (v. 2.1.0) for the online documentation site. Bootstrap 2.x was moved to EOL in Aug 2013 and Bootstrap 3.x was moved to EOL in July 2019 (https://github.com/twbs/release). Older versions of Bootstrap are also getting flagged in security scans for various CVEs:
I haven't validated each CVE, but it would probably be good practice to resolve any potential issues and get on a supported release.
The bad news is that there have been quite a few changes between Bootstrap 2 and Bootstrap 4. I've tried updating the library, refactoring/tweaking the CSS and JS to maintain a similar appearance and functionality, and testing the documentation. This is a fairly large change so I'm sure additional testing and fixes will be needed.