We will still get army themed specific Detachments that grant benefits for their armies. These will come when we start seeing Codex releases. Here is the latest..
This is from a solid source.
via Sources on Faeit 212
So "formations" are gone, but GW is still going to replace them
conceptually with detachment and army specific benefits to filling out
detachments in a specific way. While not a specific example, the new space
marine codex might have a special detachment with a tactical squad
requirement in place of a normal troop requirement, or additional command
benefits if your battalion detachment has the structure of a codex
complaint company (6 tac, 2 dev, and so on)
The formation we know today is gone, but GW still loves the idea of giving
out command benefits to fluffy armies. There will still, for some special
occasions, be some cases of "bring these specific units and gain some new
special rules", but these will be incredibly flexable to build around, and
will feel more like a hybrid of a decurion/formation and a proper
detachment.
But all and all, the days of army building by formation are over. But the
ripple effects of the decurion will still be felt once a proper codex drops
and GW gives your army more fun tools to build your lists with.
