The exemption to not inspect outstanding schools, unless there is a concern, was lifted in 2020.
Ofsted prioritized inspecting schools that were not inspected for longer period of time.
After inspection, they found the following:
1. Around 1,400 schoold that were graded outstanding were not inspected at all throughout the period which is why they kept their grade.
2. Around 1,900 were removed from Outstanding list
Read more here.