The short answer is, yes, you can make a shower without enclosing it, however I recommend enclosing on 3 sides with the shower
head pointing towards on wall. There are shower curbs that can sit flush to floor, which allows wheelchairs to easily pass over them. This
will contain the water so that you won't have to make the entire bathroom floor a waterproof shower pan. To make the shower and
bathroom handicapped accessible, you should work with a designer or architect who is familiar with aging-in-place design. There are many
things to consider and it is best to work with a professional when laying this out.