Since this feature request was declined, I am proposing something a bit more conservative:
When editing old posts and the title itself is not edited, allow the existing title to remain, even if it does contain a word-that-shall-not-be-mentioned.
While reviewing, I often stumble upon answers that are clearly follow-ups to the question by the OP. My usual response involves moving their content to the question and then flagging the follow-up as "Not-an-answer". Sometimes, though, I am unable to complete the edit of the question, unless I change the old title to something that does not contain a problem word.
I find this annoying for several reasons:
While the original title was probably not very useful, selecting a new title properly is not a task to be taken lightly. It requires clear understanding of both the question and its context. Moving in the contents of a follow-up is relatively simple. Choosing a new title, in my opinion, is not.
It forces upon each potential editor an all-or-nothing deal. I, for example, have already avoided editing several questions when I was also forced to choose a new title, since I did not feel qualified to do so.
Changing the title of another person's question, while generally acceptable in a collaboratively-edited site, is still an issue when it happens months after the fact to retroactively enforce a new rule. I can easily imagine a user using a remembered title to search for an old question of theirs to no avail.
I can just feel the urge to play word games with the original title in order to slip through the filter, rather than spend time and brain cells formulating a new title. Even if that means that the new title would be just as bad as the previous one.
I posit that, while leaving the old titles unedited does not reduce the noise ratio, it does not increase it either. So why don't we just let them rest in peace?
PS1: This is the question that sparked this feature request.
PS2: If we really want people to start editing old titles, why not encourage it separately, rather than force it? Perhaps with a new "Clean-up" tab in Review? Or even a "Janitor" silver badge?