Lyra is, to quote UE docs “a learning resource designed as a sample game project to help you understand the frameworks of Unreal Engine”. In other words, it’s supposed to show you how to use the engine, and how a game in UE can be set up. Intentionally not using "intended to be set up" because game systems and their interactions are meant to be designed based on your project.
If this is your first project in UE, it sounds to me like good advice to start with Lyra and modify some of its systems to learn how they work and interact. If you’re starting work on a new game you intend to actually finish (and not just learn UE), while you can start by just copy-pasting Lyra’s stuff, you’d end up copying a bunch of stuff you really don’t need.
As for the plugin question, I’d say it’s not a plugin because everything it uses is either base UE or already a plugin. The assets (mannequins, VFX, etc.) aren’t, but those can be easily migrated between projects.