Thanks everyone; sounds like 4.11 is too much of a leap, but 4.08 is a reasonable choice.
If anyone is using an OCaml version between 4.05 and 4.08, please do chirp up in the next few weeks. Otherwise, we’re converging on OCaml 4.08 being the new minimum version tested.
For those curious about the versions of OCaml shipped in distros other than Ubuntu and Debian, there’s a mapping in OCaml-Dockerfile.