Join Features is available with the standard analysis tools starting at ArcGIS Enterprise 10.5.1. You can read more about it here: Join Features—Portal for ArcGIS | ArcGIS Enterprise
Please let me know if you have any more questions.
Product Engineer, GeoAnalytics
Yup - you are right. The topic there was updated for 10.5 when standard tools did not yet have Join Features. That topic also covers 10.5.1 but it looks like we missed adding in Join Feature for 10.5.1. Good catch! I've just verified the documentation has been corrected for 10.6.
As an alternative, and a great way to check what's new, you can also look at the What's New documentation here: What's new in Portal for ArcGIS 10.5 and 10.5.1—Portal for ArcGIS | ArcGIS Enterprise
Let me know if you have any more questions,