My image is croped after I transform + out of the canvas + unselect : I would like it to NOT do that

The Photoshop compositor does clip off-canvas pixels when de-selected, but that can be avoided, as some have pointed out, by converting the desired layer into a smart object first (that will not get clipped, but you also must edit its contents isolated from the composition, so not a great solution), or just pre-emptively working on a larger canvas to give yourself enough room to slide stuff out of the way while you need to.

It is possible that this changed 3 or so years ago; we modernized our compositor back then and while most functionality was seamlessly preserved, this one feature may have been dropped.

I was able to reproduce the behavior and I don't think we're doing this on purpose. I found that it is tied to selecting/deselecting and not transforming. I've moved this thread to our bugs and have informed the team to look into fixing it.

I would suggest  you work in a non destructive work flow. Use Photoshop layer  create a  layer of the selected area you want to transform ( Ctrl+J) then  transform the new  layer  that way you have the image unaltered and the selected area transform in a higher image layer in the stack  nothing has been cropped or lost. Some of the transformed image  area may be clipped  be clipped the the document canvas size. However, the content is still the clipped layer's content. So it can move in or the canvas can be expanded to recover the clipped content.