I don't understand your posting. You seem to imply that archive.org is actively deleting these pages for some reason? How do you know they are "missing"? Were those links once showing the correct page? Else, how do you know there wasn't a server failure on the Slate Star Codex server which caused the ArchiveBot crawler to not record the expected page?
Note that all of those pages have 100+ copies saved, and most of them show content matching the current page.
The second of these three shows a capture of an error message from Slate Star Codex ("This is somewhat embarrassing, isn’t it?"). The other two show a capture of a 404 error. This suggests that the error is not on the archive.org side, else, why the different error messages?
The common theme is the failure captures were all about the same time, relatively recently.
Perhaps there was a server failure at the Slate Star Codex site which prevented the ArchiveBot crawler from capturing those pages at those time?
When Scott Alexander posted his final blog entry, someone on the HN thread linked to internet archive which I used to read the first 10 or so blog posts since I had never read that blog before.
Now some 6-8 weeks later I go back to continue reading and I found that any posts dealing with social issues from those first 10 was no longer accessible even though I read them on the internet archive on those same links not more than 2 months ago
June 23, 2020 (the link of the capture used in your title) was 48 days ago. The other two links you gave were captured on June 24 and June 29, so the 'common theme' is that all of the captures occurred within the period the blog was shut down.
Where is the HN comment posting showing the links you followed which once worked but no longer work now?
https://web.archive.org/web/20200618075053/https://slatestar...
https://web.archive.org/web/20200624052049/https://slatestar...