Single-switch flyback, two-switch flyback, single-switch forward, and two-switch forward converters, are all unipolar. They are all limited to 50% duty cycle maximum, in order to allow the core to reset during the other 50%. Or if driven with low duty cycle 10%, the remaining 90% is plenty of time to allow the core to reset. If you go beyond 50% duty cycle, the core will not properly reset, resulting in eventual core saturation.
Bipolar drive permits high duty cycle beyond 50% (and commensurately higher power conversion), because the core is reset via polarity reversal. But unbalanced drive waveforms will eventually lead to core saturation as well, for example 90% in one direction and 91% in the other.
Single-switch flyback, and forward if using RCD clamp snubber for reset, can go higher than 50%. Or conversely, conditions exist where unwanted DC imbalance occurs below 50%. Forward, as long as magnetizing inductance DCM or BCM is maintained, full reset is achieved (e.g. using a RCD clamp snubber for reset). Flyback can simply be operated in CCM with no consequence, as long as the control is suitable (it often isn't, e.g. peak current mode control requires high ripple fraction). In that case, the limit is not 50% unilaterally, but depends on the voltage ratio in CCM, D = Vo / (Vi + Vo).
Similarly, the subharmonic oscillation threshold that is often erroneously repeated in appnotes, is precisely the same threshold: it's simply to say current is continuous, regardless the voltage ratio. You do most of your design calculations at 50% duty, so it's not far in practice, but it is strictly wrong to claim a fixed 50% for all cases as appnotes inevitably do.
Incidentally, this means the intentional opposite of "flux walking" is simply CCM. That is, precisely the same phenomena, but actually intended. It's just total unbalanced DC bias current in the core.
Ah, yes-- not to mention that's another reason I object to the term -- it implies some ignorance of the system, some agency of it, unconstrained by the designer, that it can just wander off on its own for reasons inexplicable. As a designer, I find this wholly antithetical.
Tim