It means that McAfee is probably going to discontinue testing our DAT releases against the older engine.
DAT releases will be making use of code that's available to us in the 5600 engine; code that's necessary to identify, clean and/or delete malware that's leveraging more advanced techniques of hiding, replicating, and otherwise evading attempts at removal. Code that isn't possible with the 5400 engine, and there won't be much confidence of that code being "harmless" if used in conjunction with the 5400 engine - nothing might happen, or worse.
It's in everyone's best interest to get to the 5600 engine. If you have special circumstances that make that impossible you'll want to work with your Support Account Manager to try and establish some kind of agreement with McAfee Labs and testing of newer DATs with older engine.