Are you using the latest version of Matplotlib?

You are referring to the example from the documentation of matplotlib 2.0.2. Unless you are using that version you might rather refer to the example of the version in use. For the latest 3.0.2, that example would be found here.

In general, also consider not using the axes grid for the sole purpose of parasite axes (other applications may still make it desireable, though), but rather Multiple Yaxis With Spines.

I'm new to Matplotlib. I simply did "pip install matplotlib" on my python 3.6, and then tried the example.

There's nothing on that page that clearly states it only works with certain versions.

Agreed - its super annoying that old pages don't disappear from the main site's tree. But no one has come up/implimented a way to fix that.

Can someone manually delete all the pages in /examples? (cc @tacaswell)

The problem is that 404s all the exiting internet links out there.

One option: stick a big warning at the top of the page "This example is deprecated and for version X. Current examples are found at example.com/current."

General options are listed here: https://moz.com/blog/how-should-you-handle-expired-content

Modern matplotlib docs have a new header that has the version in it.

I think it wouldn't be a huge lift to add a banner to the old versions, at least those pre 3.0....

I'd also suggest setting robots.txt to not crawl them. If they're all in the same directory, that's an easy one-line fix.
Of course, you don't even seem to have a robots.txt, so it's a 2 line fix instead - https://matplotlib.org/robots.txt

Disallow: *
/example

And while fixing outdated pages, this just came up as a high result for one of my searches:
https://matplotlib.org/users/style_sheets.html

Oooh, there's a new version, 2.0.0rc2! :-o .... :-/

I think we would actually accept help with this. OTOH, I'm not quite clear how we make low-level chnges to the docs https://github.com/matplotlib/matplotlib.github.com

☹️ Oh, it's the outdated docs. This is (unfortunately) duplicated in various issues (e.g., #10016, #12374, etc.), so I don't think there's any need to leave this one open.