Scavenging, Freeganism, and Veganism
One area that I think there can be quite a bit of contention on is whether scavenging and freeganism are vegan. My answer to this is: it depends. I think the context in which the question is being asked is important.
My personal approach is to avoid scavenging and freeganism, and I feel this approach is optimal to the group of people in my particular situation as the optimal way to practice and model veganism. But I don't think it applies to people who live in extreme poverty or live a subsistence lifestyle where choices are greatly restricted because veganism is about doing everything practicable to avoid animal exploitation, and what is practicable to someone like me is not necessarily practicable to someone with greatly restricted resources.
Furthermore, I also think that in a context in which avoiding it is not practicable and the option is available, scavenging and freeganism is not only reasonable because it is practicable, but it literally is the vegan option. I think that practiced correctly, scavenging and freeganism do not exploit animals. I don't think all use of something that came from an animal is inherently exploitation.
Allow me to illustrate with a couple examples. If someone wanted to collect the hairs I naturally shed as I was walking around outside, I would not consider this exploitation. What would be exploitation would be forcibly cutting my hair, especially if I were treated in a similar manner as the vast majority of commercial sheep are (hyperexploitation).
A big problem with commercial sheep is they are bred to not shed their hair naturally, causing them to require shearing which their natural counterparts do not. This breeding is exploitation in itself, as it produces traits harmful to the sheep that exist only to benefit humans. Commercial sheep are then further exploitated in the most deplorable ways imaginable. Shearers are typically paid by the amount of wool they shear, not by how long they work, which incentivizes fast shearing that inevitably leads to frequent shearing injuries. Suffering is the price of profit to the capitalist class, for both the shearers and the sheep.
Consider another example. I die of natural causes or of an accident. If a human stumbles onto my rotting corpse, as long as they didn't cause my death, I don't think they're exploiting me if they decide to...eat that. Not going to spend a lot of time thinking about it since I'm not personally into vore, but I wouldn't consider it exploitation. Now, other people may feel differently, and I think that should be respected, but I don't think human conceptions of the treatment of their corpses apply generally to nonhuman animals (though this is potentially a more complicated question for animals like elephants that also seem to practice their own death rituals).
So I think scavenging can not only fit the definition of veganism for qualifying as the best practicable option, I think that in some cases it fits the definition in the sense that it doesn't cause the exploitation of any animal.
Nevertheless, I don't think scavenging and freeganism is vegan for everyone. And one big reason for this is that capitalism exists. Capitalism makes exploitation inevitable in a search for greater and greater profits. And regardless of capitalism, people can lie to both themselves and others about whether something was truly scavenged or acquired in a freegan way. They can also simply be mistaken.
For example, eating the free non-vegan pizza served at the university event isn't practicing freeganism, though it may seem to be. This is because while that pizza may be free for the person eating it, it was bought by someone who is thus producing a demand for it. On the other hand, eating a non-vegan pizza that was obtained by dumpster diving is freeganism because no additional demand is being created by that act.
I also think that if an item is being sold under capitalism, that even if people start purchasing only scavenged items, a strong incentive exists to label items as scavenged that definitively are not, in the same way that greenwashing occurs and in the same way that corporations lie or exaggerate about how well they treat their workers. Additionally, this incentive is increased given that scavenging is not nearly as scalable as more exploitive methods of production, and so practical veganism needs non-exploitive solutions that can be deployed at scale in order to effectively counter animal exploitation.
Quite simply, there is no way out of exploitation of any kind without being able to deploy solutions that can scale to fill the necessary demand, which scavenging and freeganism cannot do. And this is why, even though these can be vegan, even optimally vegan, in certain situations,they are not vegan in a general sense. Additionally, in the current environment, for some people, they are vegan only in the sense that better options are not practicable for the people in question. This is because I think it's important to show that one isn't 'too good for' ethical solutions that can be deployed at scale. Otherwise the position becomes that avoiding exploitation is somehow only for the well off, which is not itself a vegan position since that would necessarily only reduce exploitation for a small fraction of the animals currently being exploitated. There is therefore also a larger obligation, when possible, to push society to deploy the solutions that veganism needs to end exploitation at scale, and not just on an individual level.
And this also just so happens to help end the exploitation of the most oppressed and hyperexploited of humanity as well, because the hyperexploitation of animals of today is fueled by the hyperexploitation of human workers in a way that simply cannot be resolved at scale for the human workers involved without abolishing the exploitation of the animals involved.