According to Motorsport.it it has been agreed that the Formula 1 engine should not have a qualifying mode from the Italian Grand Prix onwards. The plan was actually to introduce this before the Belgian Grand Prix, but according to the Italian medium Honda and Mercedes have asked the FIA to postpone this by one race.
Honda needs more time
Honda and Mercedes would need more time to investigate the different engine maps on the test bench, to make sure there are no problems with the engine if it is only used in one position. Despite the ban being in Red Bull Racing's interest, Honda required more time.
This does create a strange situation in Belgium, because some teams will introduce their new engine in Belgium. Esteban Ocon, for example, will start Renault with a new engine, which already complies with the new guidelines. However, the teams can also choose to use the old model.