I have nothing to point to, but my experience is that enclosing terms in quotes makes no actual difference.
Google search has been getting increasingly worse for me over the last five years or so. In terms of the quality of search results, it has no edge over DDG.
I stand corrected, I wanted to provide some examples but I cannot reproduce this anymore.
From memory I do recall the +/- and "" operator being overriden if there was a "popular" search query that contained broader results than my specific query.
Specifically for the "" operator was that the query would drop "" so instead of showing x results which must include that expression it would show broader results as if the expression was not contained in "".
There was a time, several years ago now, where that was happening pretty regularly to me. It's gotten better, but since 2009 or so Google search quality has been on the decline for me. It only works well when I use things like site:blah.blah or quoted searches, or something I happened to search for and click on before (but couldn't find in history or had found on another computer). Finding novel things is a pain.
My guess is that, by now, resource intensive 'power users' are not worth the effort for google.
For technical queries I moved on to ddg, it's not perfect but a hell of a lot less time waisted.