It can be downloaded here:
The main feature is this new release is the plugin system. Its goal is to extend the application easily by dropping JS files in the "plugins" directory.
For now I've put in place a basic framework to load and run plugins and a basic API that can be extended over time depending on the feedback from developers and users. If you'd like to give it a try there's an overview that explains how Joplin can be extended, as well as a getting started documentation and the complete plugin API documentation. There's also a tutorial that explains how to create a TOC plugin. Finally some of the technical details are in the Plugin architecture document.
At this point the feature is mostly for developers or relatively technical users because there's no UI to load and install plugins and no repository to search for existing plugins. To try a plugin, you'd drop the JS file in the profile "plugins" directory as described here: https://joplinapp.org/plugins/ Later we can add a UI to do this as well as some way to enable/disable a plugin.
To make Joplin more extendable, a lot of code has been refactored or converted to TypeScript, and while all tests pass and I don't know of any bug, I wouldn't be surprised if I missed something so if you find any bug, please report it here.
Also if you try to create a plugin and run into any issue, please let me know. Also if a feature is missing to implement a plugin you have in mind, it shouldn't be too hard to add it to the plugin API.