Yes he has
improved a lot over the last couple of years, as has been broadly noted I think even by pundits who were previously critical of him.
It's also improvement you can track statistically. Aside from getting better at the defensive side of things, he has improved in other aspects of the game too. For example, he has openly spoken about improving how much he drives forward with the ball and that's something you can see getting better year on year statistically, so he now carries the ball forward twice as much as he used to a couple of years ago.
Crucially it's also what he
isn't able to do in this England side when Southgate sets them up like he did against Croatia.
Statistically he's also improved in terms of passes completed, pass accuracy,
chances created, take-ons, fouls won, passes in the opposition half, forward passes, etc.
Though as you say, you would expect improvement. Over the last few years he has played
a lot. Between October 2018 and April of this year he played more minutes than any other outfield player in the PL. And in that time he has become nailed on
as a midfielder for both West Ham and England, also captaining West Ham. And at 22 you'd expect him to keep improving.
None of which means we should sign him or that he's worth a ludicrous fee.
But he is getting better year on year.