This is my personal opinion. It should not be taken as a statement from the crates.io team, nor does it imply that anybody else on the team shares my opinion.
In general, I like this technique, although I think we need more user research to make sure this is solving the problems we hope it’s solving. Namely, as said other places in this thread, this really only addresses the “shared branding”/officialness problem. Is this a problem worth solving? Is this the best solution to that problem?
In addition to the questions already raised about sigils/naming/etc, I’d like to see more details around permissions made explicit in an eventual RFC along these lines: it seems like whoever is designated as an owner (either directly or via team ownership) of crate “foo” should be able to create new crates “foo/whatever”. Then should crates “foo/whatever” automatically get the same owners of crate “foo”? That seems like the common case and would save folks time having to remember to add the other owners, but are there scenarios where that wouldn’t be desired? If, after creation, the owners of either the crate “foo” or the crate “foo/whatever” are changed, should those changes be synced to the other crates in the group? Only if the top level crate’s owners are changed? What would folks expect?
EDIT: Also, thank you @ethanpailes for taking the time to write this up!