I think this is more Claude writing than LLM writing. GPT-5.6 also talks like an LLM, but not like this.

With GPT-5.6 I get reasonable results with "use natural, plain English". I tried "use ASD-STE100 Simplified Technical English" but I've found "natural plain English" to work better. Still not on the level I would write, but better.

In the mean time, I hear that Claude resists changing its writing style. A PO used Claude to write release notes and it was full of Claudeisms and tried changing the text into a TED talk. Then he prompted Claude to rewrite in ASD-STE100 Simplified Technical English, and Claude barely changed its writing. Then he switched to GPT-5.6 Terra, and GPT revised everything and made the text much better.