coffee on LCHF

Decaff should be fine.
When I was first diagnosed, I read up a lot on coffee, and the general line seemed to be that decaff is, if anything, preferred for T2s. I actually drink a lot of very strong black coffee every day and it never spikes me, but some do say that the caffeine can raise cortisol and BG levels etc.
I personally like the effects of caffeine, but if you don't, then I would say decaff should be a nice and safe enough thing to try.
Do bear in mind that some decaff brands still have quite a lot of caffeine left in though.

I use decaf, though I have mine with double cream and I have had no adverse effects.