I made challah dough and it was still sticky after 9 cups of AP flour

I have conflicting experience to @Benjamin. I often add a little extra starch to my bread actually, specifically potato starch or sweet rice flour. While all starches gelatinize a little differently, I would not expect you to have any issue shaping the bread, or with rise. What I would expect is a little extra chewiness to the crust and perhaps a bouncy quality to the interior, and maybe a tighter crumb with finer bubbles. The first time I added starch to a bread as an experiment (and far more proportionately), I joked that I had successfully invented storebought bread at home, because that's what the texture reminded me of. That said, I would not expect the textural differences to be huge in your case because you added very little.

Since percentages and math are on the table (I'm sorry but I love math), let's break down the amount of gluten and starch in here.

The recipe calls for 9 cups of flour total, and there are 16 Tablespoons in a cup. So how much gluten is in one of those cups? If we say it's ~12% (I asked Google), then in one cup of flour, there are about 1.92 T gluten, and about 14.08 T starch. If you round, that means about 2 T gluten, 14 T starch. That's a proportion of 1:7.... and I'm rounding up the gluten.

So what difference does a few tablespoons of corn starch make, proportionately? Well, for every T of corn starch you added instead of AP, you were missing one-seventh of a Tablespoon of gluten, or less than a half-tsp, but more than a quarter-tsp. And that amount counts up with every T you added. So, if you added a quarter-cup of corn starch, then you're "missing" almost 2 teaspoons of gluten. If you added a whole half cup, you're missing a little over a Tablespoon of it. But again, we rounded the gluten up before, so... you aren't even missing that much. This is an overestimation.

In the recipe as a whole, you have 144 T in those 9 cups of flour. That means you have 17.3 T of gluten total, and 126.7 T starch. Even if you added the whole half-cup of corn starch, changing the amount of starch to 134.7 and the total amount of "flour" to 152 T.... You've only changed your gluten content from 12% to 11.4%, which would still be within range for all-purpose. Some brands have higher gluten. Some have less. King Arthur brand's all-purpose flour has as much gluten as some other brand's bread flours. And again... I'm assuming you added significantly more than a few tablespoons. That drop from 12% to 11.4% represents a worst-case scenario.

So while there will be a difference, I'm guessing it will be subtle, maybe even undetectable if you aren't looking for it.

One thing I noticed reading the recipe is that they don't call for resting the dough. A short rest (often called "autolyse") after adding, say, two-thirds of the total amount of flour will give the flour a chance to hydrate and soak up some more moisture. That way, it will require a little less flour to get a workable dough, and your final product will be just a little softer and moister as a result. IF you make this recipe again, it might be helpful.

I'll be curious to hear how your experiment turns out (hopefully okay!)

In general, cornstarch is a poor substitute for flour when baking bread. Although both ingredients will absorb moisture, the gluten in flour plays an essential role in forming the structure of the bread. Cornstarch on the other hand has no gluten, so your resulting dough may be "weak". The loaf may not rise high enough, and it may lack the elasticity to shape and braid properly.

The amount of cornstarch you used will determine the degree of the effect. A single tablespoon accounts for 0.6% of the total dry weight of the recipe. While this may sound small, it is certainly enough that a "few tablespoons" could make a noticeable difference. After all, the difference between the protein content of all-purpose flour (~10.5%) and bread flour (~13%) is at the same scale.

In any case, it sounds like the dough is made. Shape it the best you can. I know the recipe you are attempting is for challah, but if the dough is too weak to braid, consider shaping into a loaf and baking in a tin. Bake as usual and let us know how it goes!