A prime example often cited in discussions about deep theory involves the . This opening is notorious for its sharp tactical lines and forced sequences. In several games played between Super GMs (like Magnus Carlsen, Fabiano Caruana, and Anish Giri), the theory has extended well past move 25.
While most amateur games leave "book" within 5–10 moves, some highly theoretical lines can remain in book for dozens of plies (half-moves). Longest "book opening" - Chess Forums longest book move sequence chess.com
The reason these sequences are getting longer is the rise of "second" preparation and engine corralling. A prime example often cited in discussions about
Often reaches move 25+ before a "novelty" (a new move) is played. Semi-Slav Defense (Botvinnik Variation) and Anish Giri)