I would consider feature complete with robust testing to be a great proxy for code quality. Specifically, that if a chunk of code is feature complete and well tested and now changing slowly, it means -- as far as I can tell -- that the abstractions contained are at least ok at modeling the problem domain.
I would expect code that continually changes and deprecates and creates new features is still looking for a good problem domain fit.
Most of our customers are enterprises, so I feel relatively comfortable assuming they have some decent testing and QA in place. Perhaps I am too optimistic?
That sounds like an opportunity for some inspection; coverage, linting (type checking??), and a by-hand spot check to assess the quality of testing. You might also inspect the QA process (ride-along with folks from QA).
I would expect code that continually changes and deprecates and creates new features is still looking for a good problem domain fit.