synfig/synfig is the official source code repository for Synfig Studio, a 2D animation software. The latest recorded release is v1.5.5. However, the provided materials do not list anything newly added or changed in this version. So rather than the specific changes of this release, it is better to look at the software the repository covers and what it is used for.
Synfig Studio is free open source software for making 2D animations using vector and bitmap artwork. According to the README description, it lets you create animations without drawing every frame by hand, which is meant to help you produce work with fewer people and fewer resources. The key point is that it is an animation tool that handles both vector artwork and bitmap artwork. The source code of this software is published in the official repository. The project is implemented in C++, and its license is GPL-3.0. The README also explicitly states support for Windows, Linux, and macOS, presenting it as a project usable across multiple operating systems.
This information alone is not enough to judge the changes in v1.5.5 or the current development status. Since per-release change details and performance evaluations are also not given, no conclusion can be drawn about which features improved in this version. The confirmable scope is limited to the fact that it is the official source repository for Synfig Studio and to the description of its aim of making 2D animations without drawing every frame by hand.