On the live image you mean? No, Alt+Super+s never activates Orca on a live Ubuntu Mate image, neither on 15.10, nor on 16.04 alpha 2. I've detailed Ubuntu Mate's reaction in this respect above with steps to reproduce and it was confirmed.
On an installed image? I don't know, perhaps, I haven't tried that, because right now I am not willing to install Ubuntu Mate on my system in this accessibility state, as I am totally blind.
On the live image you mean? No, Alt+Super+s never activates Orca on a live Ubuntu Mate image, neither on 15.10, nor on 16.04 alpha 2. I've detailed Ubuntu Mate's reaction in this respect above with steps to reproduce and it was confirmed.
On an installed image? I don't know, perhaps, I haven't tried that, because right now I am not willing to install Ubuntu Mate on my system in this accessibility state, as I am totally blind.