All of a sudden, out of nowhere, Git disappears. R can't seem to find it, and the Git tab in a project is gone.
I reinstalled RStudio & Git (which went to a different path location on the reinstall) and then pointed RStudio to the correct (new) Git path. Now it works. (Currently RStudio 1.2.5001, Git 2.23.0, but these are both new with the problem solved.) I'm working on a Mac Mojave 10.14.6.
However, I teach a class of 35 students where the Git-RStudio combination happens every day (for turning in assignments), and now my students have the exact same problem.