As of today Multivac APIs documentation (APIs UI) can be found under Multivac Dashboards:
This requires a login before accessing anything in Multivac Dashboards. You can use the same username as before to access without any problem. Also, you can continue using Multivac APIs and its UI as same as before. Just the URL has changed.
I came to the conclusion that https://api.iscpif.fr was not the right place to represent APIs documentation.
The documentation is only visible to those who have access to the APIs, therefore, it is only visible after you sign in.
Let me know if you have any issue finding or using Multivac APIs by creating a new post.