Haven't read the full thread, but saw some earlier posts about language being dumbed down and how that's a shame, and standards should be raised.
I'm a writer for the government. It's true we write for a 9-11yr reading age but it's not just because that's the average reading age.
If you think about the kind of things you use gov.uk for – applying for a passport, booking a driving test, doing your tax return – these are tasks you want to get done as quickly and painlessly as possible. Ensuring these services contain the most easy-to-process language is the best way to achieve that.
Literally millions of people use gov.uk every day (NHS website is another good example). Many will have low literacy levels, but others will have cognitive or visual impairments, and many will be stressed, anxious or be situationally disadvantaged (imagine you were a Ukrainian in the first days after the war broke out, trying to apply for a refugee visa in a second language on your phone while sitting on the ground in a freezing railway station with only 7% battery remaining, etc). You just need it to be absolutely straightforward and quick.
Other people might not speak English as their main language, or they may be autistic and find jargon, euphemisms, metaphors etc hard to process. If your first language is sign language, some vocabulary might not be as familiar. If you have a motor impairment and cannot use a mouse, it's much easier to navigate a web page when there are simpler sentences with fewer words.
Testing and research has also shown that lawyers, academics and other 'high-intellect' professionals far prefer simple, clear language when interacting with 'official' information, like government, health and financial services. Yet these are often the same people who accuse me of dumbing down!
Obviously language and vocabulary is a beautiful thing, and reading is one of life's greatest pleasures. But for the majority of our daily routine encounters with words, writing aimed at 9-11 yr olds is best for almost everyone.