Swift, Rust nor Go provide support for exotic targets. If you want code to work in actually "weird architectures and little known operating systems", then your only option is C (or weird dialects of "C" in some cases).
At least that was how things were around >10 years ago, not sure about the current state of things. Anyone can chime in? Do embedded vendors etc. provide proper forks of newish, modern compilers?
Even excluding extremely exotic targets, the list of platforms supported by Rust and Go is vastly larger than Swift. For instance, there's no official support on FreeBSD, and FreeBSD 1. uses LLVM for everything 2. has a userland really similar to Darwin, so there are very few excuses to not allocate even a small amount of man power to port it.
If you add third-party ports and implementations, the list of platforms supported by Go and Rust increases exponentially; see for instance Tiny Go and the ongoing port of Rust to the Xtensa-based ESP32.
> Anyone can chime in?
I work in embedded and as far as my experience goes nowadays unless you go on extremely limited environments, the embedded chips I mostly see or develop for are either ARM (v7, like stuff from Nordic or the STM), or ESP32 (Xtensa). Every once in a while I have to work on some older projects which were based on Atmel or PIC, or very very very rarely MIPS.
It's certainly not like it was before; nowadays you can buy chips like the ESP32-WROVER which have relatively lots of RAM and storage for an absurdly cheap price. They run pretty well even if you opt for using C++17 and complex libraries.
At least that was how things were around >10 years ago, not sure about the current state of things. Anyone can chime in? Do embedded vendors etc. provide proper forks of newish, modern compilers?