My point was "some of the upstream versions were older than Debian's" which means the "Debian is behind, Debian is slow" is not true in the case. That usually only happens where an upstream is either very conservative with bumping versions, or where upstream is under-maintained, neither which at first glance seem to be true of becache-fs.
That said, having dabled in rust there’s a temptation to name breaking changes to improve ergonomics because the type system is so expressive. I almost wonder if this is an argument for simpler type systems to reduce churn.
(There is a somewhat frustrating trend in rust packages for them to sit on 0.x versions for a very long time, even if they're fairly heavily used in the ecosystem and pretty stable in practice)
From what the author of bcachefs is saying, the forward moves were not a problem, it was moving bindgen backwards which broke the build, meaning the packaged version of bcachefs-tools stayed very out of date, causing him to get bug reports for known and fixed fairly serious bugs (ones which rendered machines unbootable).