There is a rumor. However, the three Fed people I have presented with in the last 30 days all indicated there is no substance to it. In each case, their instant observation was that bankers have gotten accustomed to delays in effective dates in recent years, so it is wishful thinking that this one might be delayed as well. All asserted that the Fed will be ready. (By the way, the Fed was expecting to have much more time to pull this together when they initiated the legislative process.)
However, in response to a private question from me, they also agreed that it may not even be within the Federal Reserve's power to delay the effective date. Whether the Fed is ready or not, some of the big guys will be and preventing them from going forward would be hard to rationalize. Any bank with image quality problems that flips the switch without resolving them would be assuming enormous risks - self restraint may be the most likely cause of delayed entry into the world of sending image cash letters.
As a practical matter Check 21 has little effect on small community and rural banks other than the fact that they are the ones who are most likely to receive substitute checks. I cannot imagine a rationale that could conclude it will be delayed because of them.