[pmachinery]

X-No-Archive: yes was for Dejanews, and subsequently Google, and worked, at least in terms of what they publicly displayed on their site.

For about the past 20 years you don't need an archive though, with some Usenet service providers having posts going back to at least 2003 (with people talking about 2003 being a cut off for this archive, I suspect that's where much of the content is coming from).

Major Usenet providers required/had so much space for binaries they just stopped expiring text posts, which are a drop in the ocean by comparison.