i havent heard of it, if you want to do it and feel like you need it then it might be a good idea, but to be honest you really cant prepare for an assessment day. On an assessment day they will be looking for how you interact with other, how you include others and if you are overbearing. also keep in mind what you wear and just in general your grooming standards. they do have a large number of scenarios that they could do as well. not every assessment day is the same, and be careful as they put some "fake" people that pretended to be applying for the job when really they are just there to see if you are only nice in front of the assessors or if you are genuinely nice all time around.
if you think that you do need it then go for it. but no one can prepare you for an assessment day. you just have to go for it and hope for the best.
best of luck if you do get to go to an assessment day.