My "Add a page" link just dissapeared

That was a design decision to encourage a certain workflow. I agree it can seem odd at first.

How to create a level 0 page now ?

Use the Add New button to the right of exist pages and then specify whether the page is going before or after (not subpage).

Thank you, but I don’t like this system, I will use an other plugin.

Thank you anyway for the big good work.

No problem. The great thing about WordPress is you have choices. Good luck with your site!