It would be great to have the same option which was released to serve in 2.11 Release available in Cloud.

To quickly pull up parent or child issue of an issue via JQL and search issues within Portfolio hierarchy. 

EXAMPLE:

For parent issues of an issue

• For parent issues of an issue in a specific hierarchy level in Portfolio for Jira, type issuekey in parentIssuesOf("EX-000").
• For instance, you have this hierarchy:
  • INIT-001 (at initiative hierarchy level)
    • EPIC-001 (at epic hierarchy level)
    • EPIC-002

• • • • STORY-001 (at story hierarchy level)
      • STORY-002
• To get the parent issues of STORY-001, type issuekey in parentIssuesOf("STORY-001"). All parent issues above STORY-001 will be returned, and not just the immediate parent issue at the epic hierarchy level. This means EPIC-002 and INIT-001 will be returned.

For child issues of an issue

• For child issues of an issue from a specific hierarchy level in Portfolio for Jira, type issuekey in childIssuesOf("EX-000").
• For instance, you have this hierarchy:
  • INIT-001 (at initiative hierarchy level)
    • EPIC-001 (at epic hierarchy level)
    • EPIC-002

• • • • STORY-001 (at story hierarchy level)
      • STORY-002To get the child issues of INIT-001, type issuekey in childIssuesOf("INIT-001"). All child issues below INIT-001 will be returned, and not just the child issues at the epic hierarchy level. This means EPIC-001, EPIC-002, STORY-001, and STORY-002 will be returned. 

Release link:

https://confluence.atlassian.com/jiraportfolioserver/searching-for-issues-using-portfolio-details-940678957.htmlhttps://confluence.atlassian.com/jiraportfolio/portfolio-for-jira-2-11-release-notes-942855170.html#PortfolioforJira2.11releasenotes-parentissues