Hello! Project have announced that they will prohibit jumping at concert venues from January 2, 2020 onward. 

Their official website states, "In order for all guests to enjoy our concerts safely, the staff at concert venues have been calling out and warning those excessively jumping. Unfortunately, the following situations have reoccurred. Obstructing the view and coming in physical contact with surrounding guests. Trouble among guests accompanying those actions. Guests jumping and falling, and as a result, injury to the said guest and surrounding guests. Damage to seats and equipment at the venues." 

As such, "Jumping will be prohibited at all Hello! Project concerts from January 2, 2020 onward." 

Source: natalie
Image: Hello! Project's official website