This was automatically closed by ftpmaster because the package was removed from unstable, but this still does not fix the FTBFS problem in stable.
Unfortunately I don't think a proper fix will be forthcoming, upstream has abandoned the crate in question. Afaict the only purpose this package serves in buster is to satisfy the dependencies of "librust-encoding-rs+simd-dev" and "librust-encoding-rs+simd-accel-dev" neither of which have any reverse dependencies. it looks like "librust-encoding-rs+simd-dev" and "librust-encoding-rs+simd-accel-dev" were dropped in unstable by the rust-encoding-rs upload 0.8.15-2 just prior to the buster release, but these changes did not make it into buster.
There are already 74 packages which FTBFS in stable (by my count),
Do you have a list? Are the stable release managers open to patches fixing such issues? (note that FTBFS indications on reproducible builds are IME not reliable, I have seen plenty of FTBFS on there that look like either resource exhaustion or quirks of the particular builders they are using, they also report FTBFS even on architectures where a package has no binaries in Debian)
it would be much better if every mantainer cared about their own packages.
For the most part maintainers care for their packages in testing/unstable, then when a stable release happens things get frozen. So normally packages that built successfully in test rebuilds during the freeze should continue to build successfully throughout the releases stable lifecycle. Unfortunately firefox/thunderbird support lifecycles combined with it being one of the most security-sensitive packages in Debian requires periodic updates to new firefox/thunderbird release series even in stable releases, that in turn has lead to new major versions of rustc being uploaded even during freezes and even to stable releases. The rust team has been struggling for lack of manpower, particularly experienced manpower for some time. I've been trying to help where I can but I'm still a relative newcomer to rust (though I understand the rust packaging system a lot better than I did 6 months ago). I do wonder if it would make sense to use seperate package names for the rust backports done to support firefox/thunderbird updates in stable so the rest of the rust ecosystem is not affected. ccing the release team and the mozilla maintainers to see if they have any input on this.