I noticed that Audacity (last version 3.0.5) uses FLAC 1.3.1 for exporting audio in FLAC format. Is there a reason to use this older version which was released in 2014, when the current version is 1.3.3 (released in 2019)?
Not that I’m aware of. Why? Is there a reason why updating the FLAC library should be given priority?
Historically, Audacity has been shipping with vendored versions of various libraries, including FLAC. This had the advantage that, as far as Audacity is concerned, the right library always is present, but the downsides are that updates are suddenly very difficult, it can bloat program sizes, git contributor history is lost and so on.
There are ongoing efforts to de-vendor and extract these libraries, but it’s a massive project, out of which FLAC only is a tiny portion. If you need to use FLAC 1.3.3 very soon, you should be able to do so with Audacity 3.1 if you use a custom FFMPEG export
Historically there was a lack of open source compatible, cross-platform dependency managers, so to avoid version conflicts / missing libraries, there were few options available. (“Conan”, the dependency manager that Audacity now uses, didn’t reach “version 1” until 2018, by which time Audacity was already 18 years old).