I use Swift on the Raspberry Pi and find the following packages very useful: They provide APIs to familiar systems which (Excluding GPIO) are available on Apple Systems, but where there is no Apple-provided API on Linux devices.
Alsey Coleman Miller is a developer who works on IOT devices and has built an array of Swift Packages, including this, comprehensive, package which provides Swifty interfaces to the Bluetooth hardware on Linux devices. I have built a system to control an electric gate on a private road which recognises authorised vehicles via Bluetooth LE tokens using this package and found it very useful.
Bouke Haarsma has built a fully-fledged Swift implementation of the HAP stack which powers HomeKit. This excellent package allows hobbyists to build their own (non-certified) HomeKit hardware using Swift and a Raspberry Pi (or similar). I have used this package to build a system to monitor the temperature inside and outside my house. I'm also in the process of using it to build a proxy which controls my Denon AV Amp and add it to HomeKit even though it doesn't natively support HomeKit. (I am aware that other ways to do this already exist!)
This is a relatively simple package written by Umberto Raimondi, who was vital in getting Swift running on Arm devices such as the Raspberry Pi. It allowed me to easily interface with the temperature sensors mentioned above and allows users to interact with the various hardware interfaces on the Pi, encouraging hobbyists to try hardware hacking.
(Hummingbird is amazing as well, but I think you've already got that covered.)