Since the release of Dragon Ball Super, it is confirmed by Akira Toriyama and Toei that Dragon Ball GT is non-canon anymore. But I was wondering if there was still some elements of GT that have maybe been kept by the author and which would still be considered canon in the actual Dragon Ball Super manga/anime, and notably the Super Saiyan 4 transformation achieved by Son Goku and Vegeta.
I do know that the events that led to those transformations are not present in the real series timeline, and so that they would not be able to transform in SSJ4 right now. But did Akira Toriyama confirmed the fact of totally ruling out this transformation?
He didn't confirm it, but he didn't create the design either. It was created by a guy named Nakatsuru. Up to today, in Dragon Ball, canon is considered what is based on Akira Toriyama's original ideas. The 80's and 90's movies weren't canon because Akira Toriyama wasn't involved in them. The 80's manga was canon because it was drawn by him. The fillers in the 90's series weren't canon because they weren't based on Akira Toriyama's ideas. So yes, you can say it isn't canon, that doesn't necessarily mean it is completely ruled out, though most likely it is. The final kamehameha was an idea from the video games which wasn't canon, yet it still appeared later in the Dragon Ball super series.