The dependency of glibc on the unwinder (for backtrace, pthread_exit and pthread_cancel) is a glibc packaging problem. You need to plan for replacing glibc anyway because its licensing could switch to (L)GPLv3+ (including for existing stable release branches).
However, it would be a fairly straightforward project to replace the unwinder used directly by Rust binaries with the one from libunwind. Given that this hasn't happened, I'd be surprised if Canonical is actually investing into a migration. Of course there are much bigger tasks for avoiding GPLv3 software, such as porting the distribution (including LLVM itself and its users) from libstdc++ (GCC's C++ standard library that requires GCC to build, but provides support for Clang as well) to libc++ (LLVM's C++ standard library).
However, it would be a fairly straightforward project to replace the unwinder used directly by Rust binaries with the one from libunwind. Given that this hasn't happened, I'd be surprised if Canonical is actually investing into a migration. Of course there are much bigger tasks for avoiding GPLv3 software, such as porting the distribution (including LLVM itself and its users) from libstdc++ (GCC's C++ standard library that requires GCC to build, but provides support for Clang as well) to libc++ (LLVM's C++ standard library).