Shimano have just announced 12-speed XT and SLX group sets - and we've ridden them.
Derailleurs for Shimano Deore XT and SLX
All the new derailleurs are what you'd expect - Shadow Plus. So they sit out of harms way and have an adjustable clutch that can be turned on and off for easy wheel removal and fitting. Pulley wheels are 13t and there's a bump stop for quiet running on rough trails.
What you don't get with Deore XT or SLX is a short cage (GS) rear mech. There is a 1x and 2x option for both XT and SLX, and the 2x mechs will work for 1x - but they haven't been optimised for that.
There is a SideSwing front derailleur, available in most clamp types.
Shifters for Shimano Deore XT and SLX
Just like the new shifters for XTR M9100, the new shifters use iSpec EV - although bar clamp models are also available. So they attach to the brake levers and have a 30 degree rotational ange so you can get the position just right. Add in 14mm of adjustment inboard and outboard and getting your ergonomics right shouldn't be an issue.
While there isn't a huge difference for feel in some XT and SLX parts, the shifters is not one of those. The XT shifters more closely mimic XTR, with a 2-way release lever, multi-release so you can shift more than one gear at once, and instant release which means there is just zero lag with the shifting.
SLX doesn't have the instant release or multi-release. So it means it isn't as fast or aggressive to shift, and it has a noticrably softer action at the lever compared to Deore XT or XTR. If you ride aggressively, or race, you will prefer the feel of the higher end shifters. The shifting performance of SLX is not inquestion - it just doesn't have that ultra-precise feel of the top shifters.
The left hand levers for a 2x setup are simple one lever design - and we hear they are an awesome upgrad for some fork lock outs if you don't use an under the bar dropper lever!