The number of Linux issues on bugs.swift.org is still significant. Even when code compiles on Linux, it often doesn't work without crashing, or has different behavior. As someone who has tried to port (non-GUI) Swift code from Mac to Linux, I'd say there's still a ways to go.
It's not always perfect, but it's certainly much more stable than it was a few years ago, and it continues to improve with each release. A few years ago I had to spend weeks with each new Swift release debugging segfaults and working around Foundation bugs and missing features, but lately my Mac-tested code usually works just fine on Linux (except for the occasional problem with C library dependencies or platform-specific APIs).
