--ignore-constraints-on doesn't ignore package available constraints

[sim642]

The option currently applies to only dependency version constraints according to the man page:

       --ignore-constraints-on[=PACKAGES] (default=)
           Forces opam to ignore version constraints on all dependencies to
           the listed packages. This can be used to test compatibility, but
           expect builds to break when using this. Note that version
           constraints on optional dependencies and conflicts are unaffected.
           This is equivalent to setting $OPAMIGNORECONSTRAINTS.

However the option name itself sounds more general, so it could also apply to package available constraints. In particular, it could further help with compatibility testing when the package currently has been made unavailable on certain OS-s, architectures, etc.