The ability to add context and a default value to the description system field is only available for Jira Data Center however the article is identified as being available for Data Center and Server.

A note should be added to the document to identify this limitation.

Configuring contexts and default values for the Description field

This functionality was added in Jira 8.16.x and the limitation can be identified in the Jira Software 8.16.x release notes

I searched through the release notes for the newer versions of Jira after 8.16 and found that this functionality has not been extended to Jira Server.