Is it true that it is no longer allowed to use a Maxi-Cosi booster cushion/seat without back support?

Not only Maxi-Cosi, but also independent consumer organisations advise strongly against using a seat booster without back support. Back support is essential, particularly for reasons of side impact protection, though also due to the positioning of the seat belt around the neck and chest. Booster seats and cushions without back support are not yet prohibited, but must comply with the minimum requirements of the ECE R44/03 and ECE R44/04 standards.