Will unmodified thinset stick to hardiboard, I want to apply a schluter membrane to it? I am making an shower
From what I see on their websites FAQ, unmodified is actually recommended, just mix it to be a "fairly fluid consistency". As far as sticking to hardboard, it will.
From experience, yes it'll stick.
The HardieBacker installation manual for attachment to subfloor says to "apply a supporting bed of mortar or modified thinset to subfloor." A mortar bed is often unmodified thinset, so I read it that you are okay using unmodified thinset. Modified thinset can be much stronger, so they suggest using it to help the floor performance. Mortar beds can be 2" or so thick, so it's not as important to have the additional strength from the modified additives.
The problem with using modified thinset with Schluter Kerdi is that any thinset that gets trapped between two waterproof layers of Kerdi will not cure.
So even if Hardie didn't approve of using unmodified thinset, you'd still need to use it anyway because of the problems you'll have with the Kerdi.