It means that in a future release it's going to be removed.
This allows an API developer to give people time to migrate to the new version / method of doing whatever rather than just pulling the rug out from under them. Both the new and the old versions are available for a limited time.
As for why not leave it there forever ... because there's a new, better way to do it. You can't support legacy code forever (if you value your sanity and your budget). All support has a cost (be that tech support hours, bug fixes, regression testing, etc)