The nursery GitHub org has been around for a while now. The original purpose was a place for libraries which were considered important for the community, but not ready (or appropriate) for the main org. Once concern was that the main repo would get too cluttered if there were lots of repos there.
We long ago stopped using the nursery for discovery of libraries in favour of crates.io. It is now used as just another org for Rust stuff. Although nominally things in the nursery are less mature than those in the main org, this is not at all strict. The main org has also passed the point of having few enough repos to be easy to navigate.
The downside of multiple orgs is that teams, bots, and settings have to be maintained in two places. This is extra work for no gain, and is a security risk (since we double the chance of forgetting to update something).
So, it seems to the core team that the nursery is no longer useful and it should be retired. The plan would be to move all the repos to the main org (repos which are no longer relevant or unmaintained could be moved to https://github.com/rust-unofficial), and to merge teams across the orgs. GitHub automatically redirects moved repos, so there should be no changes needed to either your bookmarks or Git configs.
Please let us know your thoughts!