There's no guarantee that software/library vX.Y.Z packaged by distro A will be identical in behavior to one packaged by distro B. Sure, distro maintainers have all sorts of guidelines, but in reality, mistakes happen, and there can be incompatibilities between the version a developer has been testing against, and one the end user is using.
Relying on feature flags is a pie in the sky solution, and realistically developers shouldn't have to be concerned with such environmental issues. Dependency declarations should be relied on to work 100% of the time, whether they're specified as version numbers or checksums. Since they're not reliable in practice, vendoring build and runtime dependencies is the only failproof method.
This isn't to say that larger teams shouldn't support specific distros directly, but my point is that smaller teams simply don't have the resources to do so.