I think the murkiness of this recent history is more a matter of misunderstanding at the time than people misremembering after the fact.
I remember believing it prevented optionality for more than common words. I can't recall exactly when I believed this or if I ever knew it had changed to a verbatim meaning, but quotes always seemed the more natural symbol for the latter. I'm sure my belief in the obligatory-but-not-verbatim meaning of '+' was supported by its apparent contrast to '-', which excluded search results with that term, since it makes sense that the opposing symbols would have opposing meanings.
And, my memory not withstanding it appears that intent and meaning of + in search changed over time.