The downtube and bar-end junction are the same junction, actually - they're both EW-RS910 ;-)
Also.. while 12-speed rim brake bikes do not feature 'wireless shifting', they also don't have a 'wart' or junction near the stem. The rear derailleur is still the only junction
.
(but yeah.. you could build a 12-speed bike with a (b) junction... but you don't have to use one)