TL;DR Use the definite article (du sens).
The long version is that in cases like this, we can ask two questions:
- Is there any reason to use a definite article?
- Is there any reason not to use a definite article?
In the sentence you cited, we see that the author has used a definite article in both cases. The two main reasons that come to mind to use a definite article in French are that it is indeed definite, i.e. something previously identified, or that it's a universal noun or type (e.g. « l'amour est aveugle »).
To my mind, either reason could apply in this sentence. « La syntaxe » can certainly denote a universal, and though « le sens » feels a little less like one, I suspect that's what's meant here. (Note that if sens couldn't be a universal, the alternative wouldn't be « non plus de sens » but a different noun that can serve as a universal, e.g. « non plus de la sémantique ».)
On the other hand, they could both be strictly definite, i.e. implicitly referring to the syntax and meaning of some sentence under discussion. We would need to see the context to decide.
Now on to the reasons to avoid a definite article. A few come to mind.
One is that after a negation, you generally delete the definite article. In my original version of this answer, I assumed this is what you had in mind since you only asked about the second definite article and not the first. That would be a situation like this:
Il mange de la farine.
Il ne mange pas de farine.
However, I believe this rule only applies to verbal complements. Your example is about nominal complements. Quel plan ? (a) de la syntaxe ; (b) du sens.
A second case is constructions that involve quantities: « une boîte de billes » or « beaucoup de farine » or « absence de sens ». We can see that « plan de X » isn't this kind of construction since it's rather a genitive that characterizes the plan.
However, a third case appears to be much closer to this one. This is the case of « type de X » and similar words. Previous questions on this suggest that there is no article (not least because otherwise the singular and plural would be easier to distinguish). But if we think hard about it, we realize that in "type de syntaxe", we aren't using "syntaxe" to say what kind of "type" it is, the way that we're using "syntaxe" to identify what kind of "plan" it is. Another way of stating this observation is that we can paraphrase the one as "plan syntaxique" but the other is not the equivalent of "type syntaxique".
Since none of those rules apply, we have no reason to delete the definite article. But any further analysis from scholars or native speakers is welcome!