I remember learning about IIIE (triple-I-efe) years ago while working with a computer vision researcher that used it to serve images generated with Jenkins pipelines.

Glad to see the project of server I used is still running well. The maintainer was a really nice person to work with too: https://cantaloupe-project.github.io/

It helped me that I was already familiar with some OGC & GIS tiling technologies, as what IIIF is doing is not too different. The image processing layer is different though, as you can zoom in, out, rotate, scale, etc..

There were several JavaScript clients, the one I used was one adopted by a museum, but I cannot recall the name now.

I have to say that I'm a little surprised that the BnF supports a framework that facilitates original image downloads and bills itself as "breaking down silos". They're kinda famous for charging fees for access and and putting restrictions on using public domain works that have already been digitized. (see e.g. the "Provision of an image already digitised" section of https://www.bnf.fr/en/reproduction-products-and-fees#bnf-ima...)

My problem with IIIF is that every JS library that implements a browser-based viewer should have a button in the viewer UI that lets you point to your profile so you can load your preferred viewer in-place rather than use whichever plugin/library the publisher went with, but none of them do.