Summary

Content by user macro was intentionally removed as part of the Confluence Cloud macros cleanup. 

Below you'll find a list of the macros that were intentionally excluded from the new editor and are being removed from the legacy editor along with the alternative methods you can use to get the same results.

Suggestion

Create a Content by User macro for the new editor.

Workaround

From the macro cleanup article:

Create a normal link to a search for the author using the following link structure:

https://site.atlassian.net/wiki/dosearchsite.action?cql=creator%3D%22USER-ID-HERE%22

The user ID is obtained from the URL of the user's profile on admin.atlassian.com.

Example:

https://admin.atlassian.com/s/SITE_ID/users/USER_ID_HERE