I believe all HA rooms
should have a fold-down bench in the shower. Separate shower chairs/stools would primarily be for someone requesting one who doesn't have an HA room. I suppose it's possible the bench breaks and the immediate fix is a separate chair/stool.
As for pictures not showing the bench, it may be folded up against the wall and hidden from view by the shower curtain.
But the PP isn't booking an HA room -- she is asking about booking a regular stateroom and requesting a chair/stool. That should be possible, but depending on how "last-minute" it could be important to contact
DCL in advance to make sure the request can be met. If not, there are somewhat portable units available that one could breakdown for travel; I don't have specific experience on how sturdy or reliable is that option.