The External MySQL database limitations is not referring to the "root" user requirement, however it still appears in the text in 10.2 (point 5): 

"The user should always be root. For more information see External MySQL database limitations."

https://access.redhat.com/documentation/en-us/red_hat_3scale_api_management/2.11/html-single/installing_3scale/index#externalizing-the-mysql-database

This applies to other versions of the doc as well.

The requirement for "root" user was originally removed as requested in this issue https://issues.redhat.com/browse/THREESCALE-6598