The word comes straight from the magenta carrier itself, and its outspoken CEO John Legere. The reason is that after you update your device it may in fact lose connection to T-Mobile’s network. That’s obviously not something anyone would want, so the advice is to refrain from making the jump to iOS 10 until the underlying issue gets fixed.
Apple is working to resolve the problem, and the fix should be out within 48 hours – so you don’t have to wait too long to be able to safely install iOS 10 on your T-Mobile iPhone 6, 6 Plus, or SE. If you have already updated, then a temporary fix for the network disconnection bug is to restart your handset when it happens. One restart won’t cure it forever, though – you’ll have to perform it every single time your iPhone disconnects, unfortunately.