Nowhere, and I criticize it every time. I've even gone to the lengths of trying to find DKIM keys for published emails in the past[1].
But at least those emails were un-redacted. To spell that out for you: in a highly-charged, highly-contentious political setting, emails were published un-redacted and theoretically verifiable with DKIM. No such possibility exists for OpenAI's blog post.
In that case, let me ask: where have you ever seen DKIM signatures being provided on a public blog post/press release?