If TAS is canon, then "The Practical Joker" would seem to settle the question of what they are. When the computer starts giggling in the rec room, McCoy says the noise was probably just one of the sound effect tapes "rewinding."
Ain't nothin' rewinds except a tape.
The mistake there is the assumption that if a show overall is canon, it means every tiny detail is absolutely binding. That's not true, since any long-running canon has bits that contradict or ignore earlier bits. Canon is something that applies in the aggregate, not on the detail level. There are going to be inconsistencies and mistakes in any long-running fictional creation, but canon is the pretense that it all fits together as a continuous whole -- a pretense that often requires ignoring or reinterpreting the bits that don't fit or don't make sense.
Heck, there are major events in Trek canon that are ignored by later canon. What ST V established about the ease of traveling to the center of the galaxy has been consistently ignored and contradicted by later shows. What "The Alternative Factor" claimed about antimatter blatantly contradicted what "The Naked Time" had already
established, and was completely ignored by all subsequent Trek canon. Same with the "faster than light, no left or right" claim in VGR's "Fury." So a canon can ignore entire episodes of a series without ignoring the series as a whole, or can ignore significant portions of an episode without ignoring the episode as a whole. A work of fiction is an exercise in pretending to begin with, so it's easy enough for later works of fiction to pretend that parts of an earlier work didn't happen after all, or that they happened differently than was pretended the first time. Fiction is capable of mistakes, but it's also capable of overwriting those mistakes rather than being trapped by them.
The Roddenberry memo that called TAS's canon status into question ceased to be binding the day he died over 21 years ago. Since then, there have been numerous references to, if not the events of TAS besides "Yesteryear," at least concepts and elements from it. It's reasonable to conclude that it's as canonical as anything else -- i.e. that it happened on the whole, but that portions of it may have been inaccurate or apocryphal. Roddenberry himself saw TOS canon the same way. When fans asked him why the Klingons looked different in TMP than they had in the original show, he told them the Klingons had always looked that way and the show had just been unable to depict it correctly due to its limited makeup budget and technology. He saw the shows he made as an approximation of a hypothetical reality, one that was capable of error in the way it depicted the details of that reality. Which is probably a good way of looking at any canon.