Thanks for the responses!
@cuviper this issue was happening on 10.13 which is ~2 years old. I'm guessing there is still CI for that?
@alexcrichton @kornel turns out I do depend on a crate which has a dependency on a sys crate. Setting MACOSX_DEPLOYMENT_TARGET does seem to set the correct LC_VERSION_MIN_MACOSX on the dependent C binary.
Hopefully this should fix the issue.
I will mark this issue as resolved once I have been able to verify this.