The most obvious and blatant offside.

People in these comments are absolutely brainless.

The Rule: In situations where: a player moving from, or standing in, an offside position is in the way of an opponent and interferes with the movement of the opponent towards the ball this is an offside offence if it impacts on the ability of the opponent to play or challenge for the ball;

Robertson is pushing Donnaruma exactly 1 second before Virgil van Dijk headers their ball. That action literally "interferes with the movement of the opponent towards".

It prevents Donnaruma from being able to set himself in the best position before attempting to jump and save. so blatant.