Add actual FutureWarning or DeprecationWarning for deprecations

[brews]

Right now we have these punk docstrings indicating deprecation. Might be good to trigger literal warnings about these. Would also make it easier to keep track of pending deprecations.

See https://docs.python.org/3/library/warnings.html.

Note the difference in target audience for choosing to use a FutureWarning or DeprecationWarning.

[brews]

Useful issue but this is stale. Unassigning myself. Happy to pick it up later if there is interest.