This isn't anything specific to uutils. When you build a Rust program that links with glibc, it needs to use libgcc to do the stack unwinding. If you look at other packaged Rust programs on Ubuntu, they all depend on libgcc for this reason. For example, Eza https://packages.ubuntu.com/plucky/eza and Ripgrep https://packages.ubuntu.com/plucky/ripgrep . If Ubuntu moves to some safe, permissively licensed glibc replacement in the future, this requirement will drop off all their Rust packages. I'm not saying this uutils change alone will let Ubuntu get out of GPLv3 compliance, I'm saying they likely view GPLv3 software in the base install as undesirable due to their IoT customers and will replace it with a permissively licensed alternative given the opportunity.
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).
