Simplify the following text for a Grade 9 native English-speaking person.
Original: In the wake of the Ibrox stadium disaster in 1971 , safety licences were required by clubs which resulted in the capacity being drastically reduced to around 20,000 .
Simplified: After a disaster at Glasgow Rangerâ s Ground , Ibrox in 1971 , ground capacities had to be reduced to around 20,000 people .