why is analysis suggesting 1. c4? It used to be 1.Nf3!

It does not matter. The initial position with 32 men has too many possibilities so the engine cannot evaluate properly because of the horizon effect.

The five best moves that Stockfish 11 suggests are mainly
(1) 1. e4 - King’s Pawn Game
(2) 1. d4 - Queen’s Pawn Game
(3) 1. e3 - Van’t Krujis Opening
(4) 1. Nf3 - Reti Opening
(5) 1. c4 - English Opening

There are 20 different moves that White can play on their first move.

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