I grew up in Boulder CO lol a lot of athletes head up there to train for any kind of running/cycling event. Like, people will train there for the Boston Marathon just cause if you can do it well up there, it'll be better wherever. It definitely makes a difference. But I don't know enough about soccer. Like do they trade people in and out like hockey or football? That'd make a big difference in how much it matters.

I happen to personally know some professional athletes (track and field not team sports tho)

They do altitude training because performing in high altitude is so rough it gives an advantage when you're back on normal altitudes. Different theories on why it works but it does seem to.