Quoting from Garner's Modern American Usage:

pleaded; pled; plead. Traditionally speaking, "pleaded" is the best past-tense and past-participal form. Commentators on usage of long said so, pouring drops of vitrol onto "has pled" and "has plead":
"Plead, sometimes wrongly used as the pret. of plead. The correct form is 'pleaded.'" John F. Genung, Outlines of Rhetoric 324 (1893)
[Seven other cites spanning the years 1905 to 1943]

The problem with these strong pronouncements, of course, is that "pled" and "plead" have gained some standing in AmE, as the Evanses noted in the 1950s (although they mentioned only "pled"): "In the United States 'pleaded' and 'pled' are both acceptable for the past tense and for the past participle. In Great Britain only the form 'pleaded' is used and "pled" is considered an Americanism." (DCAU at 372)

. . .

Still, "pleaded" is the predominant form in both AmE and BrE and always the best choice. . . .