Who?
The last singer to break Toscanini's rules, until last night of course, and perform an encore at la Scala, was majestic Russian bass Fedor Chaliapin: his Don Basilio in Barbiere di Siviglia was so powerful and just plain awesome that the endless cheering led him to repeat.
After that, 74 years of respect of Toscanini's rules: no singer was allowed to give an encore. The only exception were awarded to the chorus in 1984 ("Oh Signore dal tetto natìo" in I Lombardi alla prima crociata), 1986 and 1996 ("Va' Pensiero" in Nabucco).
But no soloists, since Chaliapin.