Officially, Z1 is not on the update list, but it may come later on, it's a small hope and personally I don't see why not considering how close in hardware terms the 1,2&3 are.
With locked bootloaders too, it's a bummer as the aosp roms really fly on the older Xperias.
Officially, software updates have stopped for:
Xperia Z, Xperia tablet Z, Xperia Z1, Xperia Z1 compact.
Marshmallow may be the last update for Z2 series, however due to the very close cpu and hardware set on 2&3 they may offer the same support as Xperia Z3
Marshmallow on Z2 & Z3 offers camera 2 ui (the Z5 camera ui) and should also have tap to sleep as well as wake which still hasn't arrived on Z5 yet!
This is the most up to date Sony version of Marshmallow and is same build as the final Beta version which was very highly praised.