I spotted John Mueller of Google giving a Reddit user advice on how to handle helping Google index content that still uses some of the old AJAX crawling scheme that Google deprecated long ago. I don’t fully understand the advice but here it is.
Google will render the #! URLs (not the escaped-fragment ones), others might still use the escaped-fragment ones. You can’t pre-render #! URLs. You could probably set up a reverse proxy with non-#! URLs that render the #! URLs and serve them, but it feels like a lot of work for a temporary fix (potentially more work than just going to a responsive design).
He then advised the webmaster to redo the site in something that Google supports now. He said “I realize making these kinds of changes is hard, but given that #!/Ajax crawling was deprecated over 3 years ago, maybe it’s time they revisited the #!-setup on the site rather than spending more time creating hacks to work around it.”
Forum discussion at Reddit.