My PC is running Ubuntu 17.10 and has GTK+ 3.22.
When compiled from this environment the binaries cannot be run under Ubuntu 16.04 since the only GTK+ 3.18 is compatible with Ubuntu 16.04. How do I compile for a lower GTK runtime level?
The other way around should work though.
Compile the binary on the older OS version and it should work fine on the newer one.
The usual solution is to not provide binaries at all, but let the users and the distribution developers / packagers deal with the problem.
Just make sure your app is easy to compile, which means use standard build system tools like mesonbuild and add some build instruction in a README.md file.