There are changes to some APIs in Exiv2 v0.27. I don’t believe we have a list of these changes. The change log documents the issues and code changes and does not focus on the API.
When you build/link darktable with the exiv2 v0.27 header files, I would expect darktable to build/link/execute without issue.
We are planning a series of “dot” releases to Exiv2 v0.27 for the next couple of years. Exiv2 v0.27.1 is scheduled for release on 31 March 2019. If you have a “show-stopper” issue with the Exiv2 v0.27 API, I will investigate restoring anything missing. I expect the Release Candidate of Exiv2 v0.27.1 to available in early March for testing.