The old context array has been replaced by the new SimplenewsSource concept. A Source is an implementation of SimplenewsSourceInterface.
To be sent through the mail spool, an implementation also need to implement SimplenewsSourceNodeInterface as currently only Nodes can be sent as real newsletters with spooling support.
The used implementation is determined by the 'simplenews_source' variable, there is currently no UI to set the value of that variable.
The default implementation additionally provides a pluggable caching mechanism. A cache implementation needs to implement SimplenewsSourceCacheInterface. There is an abstract implementation available that provides static caching which can be overridden to control which things should be cached. It's also possible to provide a custom implementation that does e.g. persistent caching.
Cache implementations can be exposed with hook_simplenews_source_info() (documentation pending), see simplenews_simplenews_source_info() for an example implementation.