Correct, the prefix, defined by the display_text_range index values in the new Tweet objects. The exclude_reply_user_ids parameter will be used to remove Twitter handles that would otherwise previously have been added by the auto_populate_reply_metadata option.
From the doc:
The auto_populate_reply_metadata option will append @mentions into the metadata as a reply chain grows, until the limit on @mentions is reached. In order to edit down the list of handles, an additional option, exclude_reply_user_ids, will enable specific IDs (apart from the leading one) to be excluded from a reply. This parameter is an optional, comma-separated list of user ids which will be removed from the server-generated @-mentions prefix.
Again note that the changes to how replies and mentions are handled are not yet live on the platform, only the attachment_url parameter is currently supported.
