I've never seen anything like this and I would laugh loudly in the face of any instructor who tried to have me sign such an agreement.
I didnt believe it when my friend told me. But he had his copy, and they used to have the document on their website. They could have done away with that policy in the last year, but Im pretty sure they have it in the first place because the head BJJ instructor learned from Helios Gracie (I dont know enough about BJJ's lineage to question any of it personally) so I;m fairly confident nothing has changed. But its just so ridiculous that they went so far with legalities, and so ironic that a Mixed Martial Arts gym doesnt let students mix martial arts