A theoretical novelty. The move is natural, as the pawn on b6 is useful to reinforce the structure after the c6-c5 move, something that will inevitably happen sooner or later. Furthermore, the bishop can be developed on b7 in some cases.
1. e2e4  1... e7e5  2. g1f3  2... b8c6  3. f1b5  3... g8f6  4. d2d3  4... f8c5  5. b5c6  5... d7c6  6. b1d2  6... f6d7  7. e1g1  7... e8g8  8. d2c4  8... f8e8  9. g1h1 [%c_effect h1;square;h1;type;Interesting;persistent;true] Not a very popular move, but it was played in two important games in 2022 by Ukrainian GM Kirill Shevchenko. GMs Viswanathan Anand and Fabiano Caruana, among others, have also tried this move in older games. The main idea of the mystery king move is to prepare the f-pawn advance and also vacate the g1-square for the white knight, which can maneuver via g1-e2-g3, as will happen in the game. Of course, it's hard to believe that this plan gives White an objective advantage, but what is sought in this line is a middlegame position that allows for the creation of imbalance.