The latest publicly released version of xournalpp/xournalpp is v1.3.8, and the source is the GitHub repository. Xournal++ is a desktop app for writing handwritten notes with a pen and for annotating PDF documents.
The repository description states that the app is written in C++ and GTK3 and that it supports Linux (Ubuntu, Debian, Arch, SUSE, etc.), macOS, and Windows 10. It also supports pen input devices such as Wacom tablets. The README describes Xournal++ as note-taking software aimed at flexibility, functionality, and speed. It handles pressure-sensitive styluses and digital pen tablets, annotates PDFs, and lets you select and copy or highlight text in the background PDF. Beyond drawing tools, it supports SVG, PNG, and PDF export, LaTeX, audio recording and playback, and Lua plugins. The license is GPL-2.0 and the repository has 15542 stars. The repository was created on August 8, 2013.
However, the material reviewed does not itemize the individual changes in v1.3.8. To find out what this release fixed, you have to look at the repository's release notes directly. The only score left in the research record is a momentum score of 3.5 (out of 10), with no novelty, technical, or quality scores given. So this record offers no basis for gauging the extent of v1.3.8's improvements. It is closer to a tool to use within that scope when you need handwriting and PDF annotation.