How would one know they are getting better at communicating clearly and effectively? Compare with code or drawing where mistakes are instantly seen. Writing doesn't have that, and if you read back your own writing you always know what you were thinking. Who has got time to read what you write, from an outside perspective, and reply with what they think you said, without getting involved in whether they dis/agree or dis/like?

Maybe there's a website in that - parrot.me where you post and other people reply only with paraphrasing what you said back to you.