Although not strictly Bootstrap, the standard kbd tag can be used to denote user input:
Press <kbd>cmd</kbd> + <kbd>s</kbd> to save.
Figure 8.14: Using the kbd tag to denote user input
Source: Jakobus Benjamin, Marah Jason (2018), Mastering Bootstrap 4: Master the latest version of Bootstrap 4 to build highly customized responsive web apps, Packt Publishing; 2nd Revised edition.