In my opinion the edit is neither necessary nor useful.
I will first discuss the usefulness. Changing the OP the question becomes trvial; it would have been easy to find an answer via a simple search on the internet. The only reason the question made sense was that OP did not remember the correct word but a similar one.
The question as asked should now even be simply false; it is not true OP searched for
"Graph theory is the slum of topology" and could not find it (they searched for something else, and had they searched for this they would in all likelihood have found what they were looking for; the point is not that it is false in a strict sense, this is accepted often to match the Q&A style, but that it is misleading in that it claims something is hard to find that is in fact trvial to find).
Having the question preserved in its original form would have had the effect that if, say, in two years somebody would have the same confusion of words as OP, which is perhaps unlikely but then not that unlikely given that the words are similar, they would easily find that question via a search and then the information; for the edited question where the info is hidden in revision history and comments this is a lot less clear.
Now, for necessary. In a comment it was said it was a "reminder of the high standards being kept on this forum" to make such an edit (the precise suggestion made for the edit was slightly different, namely to strike out "scum" and replace it by "slum", which was the intermediate version). I do not understand this. To the contrary, I would assume or at least hope, that the standards here are high enough so that everybody is able to distinguish between somebody asking (in good faith) for the origins of a potentially offensive quote and somebody saying this or promoting it.
As it stands, the question serves no further purpose, as OP got the answer they were looking for. I thus propose to either delete the question (it is unlikely it is deleted via the normal process due to score and answer), which was judged as off-topic anyway. Or, to restore it, so that it can serve some potential future puropose.
If the issue is that the title alone did not make clear this was about a quote, a solution could be to change the title to something like: 'Source of the quote "Graph theory is the scum of topology" (or something very similar)'