In DW the rules are clear: if you do it, you do it - and a player-side move is resolved - and otherwise the GM makes a move, soft unless the conditions for a hard move are satisfied.
The function of custom moves in DW, AW and similar games is not to "allow for things the rules don't explicitly allow", because there is no action that is feasible within the fiction that the rules don't explicitly allow.
Analysing what is / is not covered by rules in PbtA is interesting for several reasons.
I think about cases such as - without multiclassing, can I play a dwarf wizard? If so, what's my starting move? I haven't yet spotted rules to cover this... but then I'm not nearly as familiar with the DW game text as I am with others.
Assuming it's a lacuna, one way of addressing that is to say that DW is closed. Not only the system, but the
fiction, just doesn't contain any first level dwarf wizards. I read a DW player proudly describing their dwarf bard, which makes me feel that some groups imagine a world where there are first level dwarfs of classes other than cleric and fighter.
I believe there are a host of - what I'll call -
structural rules where doubts can lie. One such case I've seen folk express doubts about is figuring out the precise implications of rules for rations. There are many places where a move requires DM to dial-in a parameter e.g. for rituals how much exactly is "a lot of money" and should that scale with effect power?
Another category of doubt is that much as I appreciate that doing something in the fiction that doesn't invoke a move is resolved in view of positioning (a canonical example is prising a ruby from a statues eye: GM rules that it just happens) these resolutions seem quite difficult to crisply differentiate from GM decides.
Thoughts like these make me wonder what each poster means by complete versus incomplete? In the context of this thread I take it to mean that the rules do not state precisely what comes next so that from a given game state two GMs G and G' might say different things. It's incomplete because some additional principle, unwritten rule, or thought is guiding them to their (differing) answers.
One can say - well, it still falls within the defined game processes. Slouching then onto the stage is that rough beast, rule zero.
Anyway, perhaps debate is needed on what is meant by incomplete before it would be possible to agree that a given game text has / does not have that quality.