Written case interviews are indeed becoming very fashionable nowadays as a way to interview!
In this case in particular, and given that you only have 1h, I would focus on 4 slides, 5 tops. More would probably make them be less well.
Remember that the skillset tested is the same than in the "usual" cases, hence, all the practice you may have done totally plays in your favor.
One important point to add is the need to be very 80-20, structured and to the point, since the prep time is very short, so we need discipline with the analysis to have enaugh time to prep the communication strategy.
There are many many entries in thsi same Q&A regarding written cases, hence, I would recommend you to look with the keywords "written case"
Hope it helps!