I corresponded with Flarm a few months ago about this and after a couple of clarification replies I think that I understood what they were doing:

Major updates (currently V6.xx) we know about and need to be loaded by the specified date and are not interoperable with earlier major versions.

Minor updates (e.g. all V6.xx) are interoperable but each one stops working 2 years after release (or when a major update is required if sooner). The logic is that if all owners do what Flarm recommend and update with the currently available software annually at glider inspection time then, no matter when in the year they do so, that version will operate, and be interoperable with every other Flarm following the same guidelines, for at least a year (major updates excepted). If we forget to update software altogether the Flarm unit *will stop working* 2 years after the release date of the installed version - or at the next major version change.