Multi class was confirmed on release additionally there is suppose to be mercs with live release so why restrict yourself to 1 class / race combination instead of a full party option. With speculation of level cap being 12 currently
Party 1
Half-Elf - 3 Hexblade / 9 Oathbreaker Paladin ...
Hexblade was published in Xanthar's Guide to Everything.
It is unlikely to be included in BG3.