This borrows heavily from how we deprecate features in Neat using Sass @warn:
thoughtbot/neat@5c337cc

One key difference from Neat is that we only want to use this mixin for
deprecations, not all warnings. The reason is because with CSS, it's not always
a straightforward task to upgrade libraries or frameworks; particularly major
releases. So we offer a toggle to globally disable deprecation warnings, but for
other meaningful warnings we need to throw, we still output those.

To deprecate a feature, you include the `_bourbon-deprecate` mixin to throw a
Sass warning, providing a detailed message that gives context.

We offer a variable to allow people to disable deprecation warnings from being
output.

One problem with this system is that we cannot use these deprecation mixins to
deprecate a function. This is because Sass doesn't allow you to include a mixin
in a function. For now, since we only have a few functions up for deprecation,
we can live with this and manually write warnings for those.

Add deprecation warnings for features that will be removed or changed in 5.0.0.

For more information on why most of these features are being removed, see:
#702