I've made a lot of search, and I think I've finished my quest :
I've contacted the author on github, and he explains publicly the reasons why this nice project have been stopped . I authorize myself to publish here a part of his response as it is in a public thread he response to me :
The reason for removing the repository is that Grav uses Markdown for content and the editor ContentTools can't work with Markdown. ContentTools requires content to be stored in HTML. Grav can do that too, so that is why it all was working fine.
The problem was in that many people were very confused about the switch from Markdown to HTML when using Grav. All Grav documentation focuses on Markdown which becomes useless when you switch to HTML.
Also I could not get an image upload function working from the ContentTools editor, which is, I think, required.
I agree that ContentTools offers a very nice user experience and I would love to see it back with Grav.
to see the integrality of our conversation and if the link he's gave inside is always valid, download the content of his plugin : click here