What bendy-lib does
Bendy-lib is a library by KosmX for bending model parts. It's built around cuboids, the boxes that make up Minecraft models. The API lets a mod swap one of those cuboids for a bendable version.
On its own it does nothing. Install it and you won't see a new item, block or mob. Its whole job is to give other mods the bending ability.
Who it suits
Mainly mod developers. The author says it's light and that developers can include it inside their own projects. Players won't get anything from it by itself. The project page files it under Libraries and APIs, and the license is MIT.
How the API works
The README describes swapping a cuboid with priorities. That's there so several mods can work on the same model part without stepping on each other. Here's the thing, the bending style follows Mo'bends.
To swap a cuboid, you write a class based on MutableModelPart and implement its methods. You can use the existing bendable cuboid objects or write your own. The author recommends the existing ones, since building bendable geometry from scratch is a lot of work.
For Gradle, the README shows the dependency declared with modImplementation and also with include, pulled from Maven Central. It says the latest version can be found in GitHub packages. The repository also has a test mod, an older and modified version of Emotecraft, and the release branch holds the source of the latest release.
The author admits the documentation isn't finished. Questions can go to KosmX on the Fabric Discord server or the Emotecraft Discord server.
Compatibility
The Modrinth page lists Fabric, Quilt and Forge as supported loaders. A listed Minecraft versions are 1.20.1, 1.20, 1.19.4, 1.19.3, 1.19.2, 1.19.1, 1.19, 1.18.2, 1.18.1, 1.18, 1.17.1, 1.17, 1.16.5 and 1.16.4. Those are separate entries, so don't read them as a blanket range.
It's marked as required on the client side and unsupported on the server side.
One more thing to keep in mind. The GitHub README calls it a FabricMC library, and its setup example is a Fabric style Gradle one. That Modrinth page is what lists Quilt and Forge. Check the mod page for your loader and version before you build on it.