What about indigenous peoples who hunt?
The standard formulation already accounts for necessity and availability. The question is also rarely asked by anyone in that position.
Many indigenous peoples have hunted for thousands of years as part of their culture, their spirituality and their survival, often with deep respect for the animals they kill. If veganism treats killing animals for food as wrong in itself, it must condemn these communities too, which puts a mostly Western, urban ethic in judgement over peoples who have already had their land and ways of life taken from them. That suggests the moral case against eating animals is not universal at all, and depends on who you are and where you live.
Subsistence hunting by communities with limited alternatives, often following displacement from their own land, is a materially different case from buying industrially produced meat in a supermarket. The differences in necessity, scale and power are not incidental, and the 'possible and practicable' qualification addresses them directly.
There is a serious discussion here about how animal ethics interacts with indigenous sovereignty and food security, and some advocacy has handled it badly. But that discussion is not what is usually taking place. The question is generally raised by someone with full access to alternatives, about people whose situation does not resemble theirs, in order to avoid the case actually in front of them.
Meeting a criticism by raising a different issue instead of answering it. A form of tu quoque used to change the subject rather than to rebut.
How it applies here: Raises a different and considerably harder case in place of answering for the one at hand, which is the speaker's own supermarket.
Replacing someone's actual position with a weaker, distorted version, then refuting that instead. The original argument is left untouched.
How it applies here: Attributes a universal prohibition binding in all circumstances, which the standard 'possible and practicable' definition does not contain.