It’s not included in core. You can find the topic here: Discourse Akismet
But the topic is closed and hidden. I’m not sure what that means. It would make sense if the plugin were no longer maintained, but this topic says it’s only removed from two hosting tiers. So why shouldn’t self-hosters and hosted customers on higher tiers be able to find the information by searching Meta? I would wait for an answer to that before installing it.
And I think one time in the first post accidentially ‘Starter’ instead of ‘Standard’ was replaced with ‘Pro’.