For the activity stream, we use to be able to filter the API result with the username parameter. But since the changes for GDPR Deprecation notice and migration guide for major changes to Jira Cloud REST APIs to improve user privacy

The username parameter has been removed from the API and we no longer have the ability to filter the result by username

Old behavior

The API can be filtered using the username as below:

https://<instance name>.atlassian.net/activity?streams=user+IS+<Username>&maxResults=100&os_Basic

Suggestion

Ability to filter the API by account ID for instance

https://<instance name>.atlassian.net/activity?streams=accountId=<Account ID>&maxResults=100&os_Basic