T-Mobile is launching 5G in 6 cities on June 28th alongside the Galaxy S10 5G. The cities are NYC, LA, Las Vegas, Dallas, Cleveland, and Atlanta. If you’re in one of these cities, you’ll be able to purchase the phone, though T-Mobile makes it clear that its 5G coverage is currently extremely limited and will only reliably work outdoors. They even kinda have coverage maps.
NYC has the largest deployment out of all of them and it's because their macro site density is insane here. The rest of the cities don't look too good.