Who should be responsible for the fade at the end of a song - the producer, mix engineer or mastering engineer?
Now however there will usually be a separate mix engineer, and mastering has become a creative process in its own right.
The question is, "Who knows best how the finished work should sound?" Since the producer works with the band (among whom are probably the writers of the song), it seems appropriate to say that the producer knows best. The mix engineer and mastering engineer are there to put extra layers of gloss on the finished product.
If the producer is absolutely certain that there needs to be a fade, then he or she can incorporate that into the multitrack, and the mix and mastering engineers will have no option but to go with it. Alternatively, there may (and should) be communication with the mix engineer and the fade can be done in that part of the process. Likewise, the fade could be held over until mastering and the producer's wishes passed on.
Technically, doing the fade in mastering is better, but there are so many options of when the fade starts, when it ends, the shape of the fade curve, perhaps even a change of EQ balance during the fade. Really, the producer, with the band and songwriters around him, should be deciding these things.
Unfortunately there isn't a definitive answer here and the person who is to be in charge of the fade would be decided by A&R or by communication between the producer and the mix and mastering engineers.
What's important here though is that the fade isn't just left up to chance, as it will be if the people involved in the process don't talk to each other.