Just one comment on this issue. The page from the Internet Archive's cache of Google's help page on the "+" search term which Thomas P. cited comes from 2009. That particular page happens to mention that "Putting double quotes around a single word will do the same thing."
>Google ignores common words and characters such as "where" and "how", as well as certain single digits and
>single letters, because they tend to slow down your search without improving the results. Google will indicate if a
>common word has been excluded by displaying details on the results page below the search box.
>If a common word is essential to getting the results you want, you can include it by putting a "+" sign in front of it.
>(Be sure to include a space before the "+" sign.)
>Another method for doing this is conducting a phrase search, which simply means putting quotation marks around
>two or more words. Common words in a phrase search (e.g., "where are you") are included in the search.
(This is followed by a couple of examples.)
Notice the lack of mention of double quotes being equivalent to "+".
So unless the use of double quotes being equivalent to "+" was a hidden feature back then, I would say that that feature was itself a recent innovation which Google quietly slipped in; and having added it in has now decided to make it the one and only way by remove the old "+" feature.