This seems to have fixed it for me as described below in another discussion:
https://discussions.apple.com/thread/4196363?answerId=19227613022#19227613022
I went to Safari.
I opened Preferences.
I chose "Tabs."
When offered the option "Open pages in tabs instead of windows," I changed “Automatically” to “Never” as described in the link above.
Apparently, in the latest version of Safari, if you choose a link that takes you to a new site, you don't get a new Safari window (As you know, when you open a new window, it has no "history," so you automatically know that you can't click the back button). But because this new version of Safari opens the link in the same Safari window that you already have open, you instinctively expect to be able to click "back." But in reality, you're in a window now that has no history and your old Safari window can only be accessed via the History button. (At least that's how I think it's working. I'm no computer whiz.)
On one of the sites that I went to, it still resisted letting me use the back button exactly the way I used to. However, at least the back button was black instead of gray and when I held down the back button, it gave me the option to return to one of several pages, including the one I had been using previously.
Switching Automatically to Never seems to have (almost) returned things back to what I was accustomed to. Way, way better than having to resort to the History pulldown menu every time.
Hope this helps.