According to the Los Angeles Times obit, Allen was the first film editor to receive sole credit on screen. That was for Bonnie and Clyde. (I'm not sure if that also includes film editors of movies made away from Hollywood.)
Among Allen's other notable films were Robert Rossen's The Hustler (1961), Elia Kazan's America, America (1963), Paul Newman's Rachel, Rachel (1968) and Harry & Son (1984), and Arthur Penn's Alice's Restaurant (1969) and Little Big Man (1970). (Penn's Night Moves and The Missouri Breaks were less successful collaborations.)
Also, Sidney Lumet's Serpico (1973) and his expensive flop The Wiz (1977), John Hughes' The Breakfast Club (1985), and Robert Redford's The Milagro Beanfield War (1989).
Now, although Dede Allen became an important name among film connoisseurs, she was hardly the first editor to have her (or his) work discussed on a “serious” level.
Even if one ignores the montage experts of early Russian and German cinema, and stick to Hollywood fare, there was the much-discussed editorial work done on the chariot races in both version of Ben-Hur (1925, 1959), Robert Wise's editing of Orson Welles' Citizen Kane, and George Tomasini's work on Alfred Hitchcock's Psycho (1960), to name four instances.
Also, it's hardly as if Allen's editorial touches were developed in a creative vacuum. Both The Hustler and Bonnie and Clyde were very much influenced by the British, French, and Italian new waves, which boasted innovative narrative structures such as those found in Michelangelo Antonioni's Il Grido (1957), Tony Richardson's Look Back in Anger (1959),and Jean-Luc Godard's A bout de souffle / Breathless (1959). (Never mind Pauline Kael's remark that the very Nouvelle Vagueish Bonnie and Clyde was “excitingly American.”)
Also worth mentioning is that in addition to creating visual edits, Allen made use of sound for dramatic effect, e.g., letting dialogue and/or background noises roll into or at times predate the next scene.
“She was just an extraordinary collaborator, and in the course of editing [Bonnie and Clyde], I came to develop confidence in Dede,” Arthur Penn told The Times on Saturday. “Indeed, she wasn't an editor, she was a constructionist.”