Using search term "site:eab.abime.net" on Google shows exactly how many pages are indexed at the moment.
And according to the result, the issue can not be robots.txt.
At this moment it got some of the forums like
https://eab.abime.net/forumdisplay.php?f=37,
and some attachments links, but no threads.
If robots.txt was the issue, these would have been gone with the rest.
The strange thing with the result is that it claims to have "About 94 results" but will show only 17 links.
One explanation might be the reoccurring issues with long periods of "The server is too busy at the moment. Please try again later." have triggered a severe reduction in Google systems internal quality ranking of EAB.