Here you can find a history of changes across different PLUMED versions. We mostly add new features without breaking exising ones. However, some of the changes lead to incompatible behavior. In the Change Log we try to give as much visibility as possible to these changes to avoid surprises.
We also log changes that are relevant if you are developing the code. These change lists are however not complete, and if you want to put your hands in the code and maintain your own collective variables we suggest you to follow the development on github.