It is hard to make this requirement because then there is a whole set of ideas that cannot be referenced. It would close off the community that imposed that rule. In the past people have cited Geoffrey Hinton's slides for a concept origin. I don't think it is good to limit citations. Credit is important.
But it would be nice to see some rules on forums that say you cannot post paywalled items there. It would hopefully give more incentive to authors to not publish paywalled articles.
But I think a reviewer strike is a better solution that can result in a complete shift away from paywalls. The market is exploiting free labor, so without the free labor the business models must change.