This is a decent guide, but if you are interested in Swift on ARM in general, the crux is uraimo's Swift on ARM project [0], which is a huge resource for the community.
I kind of was expecting this to be bare-metal Swift running on ARM with a Raspberry Pi, when I read both the OP's title and your comment. But I was disappointed to see that this was running on Linux. From the README.md...
> Scripts to clone, configure, patch and build Swift 5.0.2 on Linux ARM devices.
Hopefully in the future, Swift would support bare-metal environments just like how Rust and Zig does.
And you can still SSH into your "bare metal" box, run gdb, etc. Best of both worlds.
Configuring the Linux kernel just to meet the requirements for bare-metal applications might work for some use-cases but not for security and safety-critical applications.
For instance, the IoT industry still suffers from typical security issues in the drivers and firmware due to these Linux/Busybox vulnerabilities in the system. There are alternatives that use safer languages or formal verification that are more secure than configuring Linux for IoT or bare metal development, like TockOS [0] (using Rust) or seL4 microkernel (a formally verified microkernel).
So, is bare metal almost entirely unnecessary in 2019? Security wise, I'd much rather use safer languages like Rust for bare metal in 2019, than to use Linux/Busybox for this use-case.
[0] - https://www.tockos.org/