The Big N hasn't stopped the supply for new firmware updates. It just takes forever to get shit from Nintendo and they always short stores.
I would know, I was manager over electronics. I would have to order more 3DS's on a regular basis. It didn't matter how many I ordered, I would still get like 4 of them in. And they always took a few days longer than my other orders.
As well that would be horrible marketing for Nintendo to intentionally short a supply for firmware updates. Not to mention if they were doing that, you should have seen the store floated when 11.2 was released.
Nintendo isn't shorting the supply for 11.3. If anything the stores in area are most likely dealing with crappy management. You have to order the product for it to be on the shelf. If you aren't ordering it, Nintendo won't send it to you.
Seriously, understanding this shit was literally my job