But as you’ve pointed out, De Gea’s decision is impacted by the player stepping over the ball because it could be deflected at any moment. That was not the case for the West Brom goal so it is not remotely similar.
Yes, but that's a separate point that isn't relevant to the line of sight argument, which is what I was using the Siggurdson to illustrate. They're two very different incidents, so the point of the comparison was just to show how line of sight is determined.
To your point though, let's imagine that instead of going through Siggy the ball instead went to one of his teammates at the back post who scored. The referee/VAR
would at that point have to decide whether Siggy's presence in an offside position had interfered with De Gea's ability to play the ball. However, it is always a subjective decision. They
might decide that because the ball travelled the entire way across the box to the back post, De Gea was in fact unimpeded in his attempt to play the ball. Or they might not. And other refs might or might not decide differently.
Relating that to the West Brom/Liverpool goal, it is also a subjective decision they have to make. However, in this instance they were unlikely to decide that the goal should be allowed to stand and
extremely unlikely to overturn the decision to disallow it.
The key reason for this is what we mean by the offside player impacting the goalkeeper's ability to play the ball. If the offside player's presence means that the goalkeeper's ability to see the ball,
or ability to judge the situation,
or ability to making a decision,
or ability to react
may have been (as distinct from
was) impacted by the player standing directly in-between him and the ball, that's enough for the goal to be disallowed. It doesn't matter if the offside player had no impact on what he physically did, it doesn't matter if the goal would have been scored anyway.
To put it another way: if having to look past Phillips to see the ball
might have intruded even slightly on the goalkeeper's reading of what was happening front of him (even just as the most minute of distractions), the goal can be disallowed, as his ability to
attempt to play the ball has been impacted (because his decision/judgement/reaction has been impacted).
And it would be extremely difficult for anyone to argue that at the moment of the header, when the goalkeeper is unsure where the ball is going, that the offside player he was having to look past to see the ball can't have been even a slight factor in his assessment of what was happening in front of him. He
had to be aware of him, because he was standing directly between him and the ball.