It is, and blessedly it actually works in my experience (on Raspbian), but it doesn't appear to be actually documented anywhere on swift.org, leaving it to the mercy of third parties like buildSwiftOnARM, whom are doing an admirable job but are still quite some ways away from
sudo apt install swift - which is really the minimal level it has to get to before the broader developer community will even consider Swift, as opposed to just enthusiasts as today.
If this sounds strange to you, imagine that you bought a new Mac and/or iDevice and were told that while it comes with Objective-C, C, C++, & Python, if you want to code in Swift you'll need to search the web for discussions about installing it, hopefully eventually find a random Github repo or four year old blog post describing how, which involves downloading gigabytes of source code, dozens of steps, many hours of waiting, and probably doesn't go smoothly much of the time.
There's no way 99% of people would bother even trying Swift, let-alone commit to it to build their production software. And that's the situation today, on Linux (and, from what I hear, Windows).
I expect I'm preaching to the choir here in these forums, to a large degree, but I wanted to lay out the situation in case some readers just aren't familiar with it (most Swift developers today being exclusively Mac or iDevice developers, after all, who simply never have to deal with any of this).
I wish I had any kind of 'in' or expertise with Linux package managers, but alas I don't. I don't have a lot of free time, but I'd at least like to know how I might be able to help with all this in a concrete way.