Rust has Editions, so deprecated features can slowly age out of the ecosystem. People using, say, the 2015 edition will probably eventually want features introduced in the 2018 edition, and they will have to remove their usage of features that were dropped in the 2015->2018 edition bump. And if they never want any new features, maybe because their library is totally complete? Well, crates using 2018... can still depend on them, they just can't use those features.