Surprise, surprise. The Sony Xperia Z3 (D6616) for T-Mobile US has a locked bootloader. We didn’t really expect any different given that all of T-Mobile’s recent Sony Xperia devices have a locked bootloader, however we thought it was worth flagging in case anyone may have thought differently.
The inability to unlock the bootloader will mean T-Mobile Xperia Z3 owners will not be able to experiment with custom ROMs. Even the ability to root the handset may be limited and is dependent on an exploit being discovered that works on a locked bootloader. The Xperia Z1s for T-Mobile did manage to get root, but only months after its release.