Mercifully, the recent vogue for making a statement with which the speaker does not really agree and canceling it by saying "Not" appears to have crested and subsided. Not particularly witty in the first place, it quickly became monotonous through parroting. Surely no one mourns its passing.*
Today, by chance, I discovered that it was not all that original. From the opening of Rex Stout's Nero Wolfe story "Immune to Murder," published in 1955: "'A fine way to serve your country,' I told him. 'Not.'"
No doubt research would turn up even earlier examples of this feeble device.
And by the way: If any hopefully-scorners are still reading this blog, note the opening sentence. The sentence adverb mercifully modifies the subsequent clause even though there is no human agent capable of dealing mercy. That is the same way that hopefully works as a sentence adverb, so lay off.
*Now "gag me with a spoon" from the Eighties, there was a catchphrase that had some oomph and could still be applied usefully in many circumstances today.Pity that it has faded.