Here is the exact wording of the new technical regulation on brake-by-wire:
11.7 ERS brake valve:
The pressure generated by the driver in the rear brake circuit may be reduced by the use of an ERS brake pressure reducing valve. The valve must be manufactured by an FIA designated supplier and installed in accordance with the fitting instructions which may be found in the Appendix to the Technical Regulations.
Any such valve may only be controlled by the control electronics described in Article 8.2.