[euroclear]

Usenet was public and distributed.

Once you posted something, you had no real control over it being retained and archived. There was a message header you could set ("X-No-Archive: yes" IIRC), but it could not really be enforced.

[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.