Media reports are increasingly indicating that all of Apple's new iPhones launching in October will feature 5G, but only the high-end "Pro Max" variant will support the fastest, mmWave technology. Fast Company says sources in the wireless industry recently told the publication as much.
This makes total sense because mmWave 5G is deeply flawed, and as a result carriers have largely lined up behind mid-band 5G to cover the bulk of their upgraded networks, with mmWave to offer boosted speeds in limited urban areas. T-Mobile isn't going to offer mmWave at all. Based on early tests, mid-band 5G is expected to offer modest speed improvements over existing LTE.
Apple analyst Ming-Chi Kuo reported back in January that Apple will release new iPhones with both mid-band and mmWave antennas.
mmWave 5G sucks — The problem with mmWave technology is that the frequencies can only travel short distances and struggle to penetrate walls, whereas mid-band 5G's reliability is comparable to existing LTE technology. Because mmWave frequencies can only travel less than a mile where mid-band 5G can reach several miles, building out a mmWave network requires more radio towers and therefore costs much more to build. Consider also that mmWave 5G drains battery life faster and requires more space inside the phone.
Your phone will still be fast — There's also simply diminishing benefit once you reach mid-band 5G speeds. Theoretical download speeds can range from 100 Mbps to as high as 900 Mbps (though since these are theoretical, expect lower actual speeds). Even at the low-end of that scale you're going to be able to download videos quickly and stream games from Microsoft's xCloud service without much issue. mmWave 5G can reach theoretical speeds of 1.8 Gbit/s on AT&T's network, but even if you can hit those speeds (you usually won't), it's hard to see how that will really make a difference in day-to-day use.
There are, of course, always going to be people who want the best of the best, and Apple is apparently going to offer it to those willing to shell out the extra money. But mmWave 5G is not good in the U.S., and Apple doesn't tend to incorporate new technology into its phones until it's mature and doesn't hurt the user experience. The benefits have to outweigh the costs, and here they really don't. So most iPhones won't have mmWave.