The LSG talked about this today. It's always been a goal for reviewers to be able to experiment proposals, and we agree that the review process often makes that unnecessarily difficult; that's something we'd like to improve. However, we're concerned that requiring a sample project would substantially increase the burden on proposal authors. Examples embedded in proposal documents are often incomplete and "didactic", i.e. designed to demonstrate how the proposal works in the language rather than to present a compelling real-world use case. Writing a sample project that meaningfully exercises a feature and shows how it would be used in real code can be a lot of work. If authors want to take that on, that's great, and we'd like to feature that prominently in the review; but we're unwilling to take the step of requiring it. We're also concerned about the suggestion of permanence that would come from putting these things in the proposal document; this is a convenience offered to the evolution community for the duration of the review, not something that should become a permanent maintenance burden for the author.
What we're going to try is adding a "Try it out" section to the standard review post. That will link to a toolchain that people can use for the duration of the review, and if the proposal author wants to offer a sample project (as a standalone SPM project hosted in their own github repository), that can go in there, too. We think putting that in the review thread rather than the proposal document carries the right implications about how long it's expected to continue to work.