Hi Nemo,
Looks like they scrapped the DjVu and "Text PDF" format, I don't know whether intentionally. You should ask in their forum: https://archive.org/iathreads/post-new.php?forum=texts (be kind!).
Just saw this forum post in IA. It seems IA is going to stop creating djvu files soon. https://archive.org/post/1053214/djvu-files-for-new-uploads
Regards,