It was a Rails monolith; one of the larger ones in the world to the best of our knowledge. We (long story greatly shortened) split it up into about ten separate Rails applications. Each had their own test suite, dependencies, etc.
However, they lived in a common monorepo and were deployed as a single monolith.
This retained some of the downsides of a Rails monolith -- for example each instance of the app was fat and consumed a lot of memory. However, the upside was that the pseudo-monolith had fairly clear internal boundaries and multiple dev teams could more easily work in parallel without stepping on eachothers' toes.