Paul has described the technical details behind this phenomenon, but
to be more explicit: it is not pip that breaks older packages, but the
new PyPI server (pypi.org instead of the old pypi.python.org) that does.

So no, there is not a legacy mode in pip. Furthermore, you won’t be
able to install the package now, even if you have the old pip version.

The only way to overcome this is to find the original package, and
either upload it to PyPI, or serve it yourself on your own server.