The notebook runtime determines the Python libraries you have access to when you open an ArcGIS Notebook. With each release, we add new libraries, deprecate old ones, or update the versions of the existing libraries.
You can review the existing runtime v9.0 and previous runtimes, along with the release of ArcGIS Enterprise or ArcGIS Online that they are included with here.
As with some of the Python libraries included in the runtime, the runtimes themselves do get deprecated over time. For ArcGIS Online notebooks, each runtime is supported for 2 years after its release. For ArcGIS Enterprise notebooks, each runtime follows the ArcGIS Enterprise Life Cycle of the version it was released with.
Now: what should you do to make sure your notebook runtimes are up to date?
Steps for changing/updating the notebook runtime can be found here.