Marking API's as obsolete or as experimental
Often times your API in your program or library evolves. So you will need a mechanism of telling that a specific API (an interface or just a simple method call) is obsolete and might be not there anymore in the next major version.
Also it can happen that you have a preview version of a API, which might not be rock-stable and the API-surface might change. How do we indicate that to the user?