Last week, CD Projekt Red released MODKit for The Witcher 3—a tool that "enables Witcher community to modify several aspects of the game to their own liking." It's a mod tool, but it's not the mod tool. MODKit lets The Witcher 3 community modify the look of in-game assets, insert new models in place of existing ones, and edit existing script files.
It allows for some light editing, but nothing to the scale of REDKit—the powerful mod tool released for previous Witcher games. Put simply, where REDKit allowed modders to create new things, MODKit only lets them edit existing things.
Asked if REDKit was a possibility for The Witcher 3, CDPR community lead Marcin Momot had this to say:
@MeerMusik This is the mod support for the Wild Hunt. At this moment we are not planning to release anything else. August 14, 2015
Unsurprisingly, this hasn't gone over too well—with many have pointing to an IGN interview from January 2014. At the time, CD Projekt co-founder Marcin Iwinski said, "we are looking to release the REDkit sometime after the game launches."
Of course, 'looking to' doesn't mean 'definitely will'. Moreover, Momot's tweet—and the phrasing of "at this moment"—does leave open the possibility for REDKit support in the future. For now though, what we've got seems to be all CDPR is prepared to confirm.