To supplement what @Walter Buggenhout stated, here is the correct URL for the JSWCLOUD issue's reference on the additional article - https://community.atlassian.com/t5/Jira-Software-articles/What-s-new-in-the-Roadmap-view-I-m-so-glad-you-asked/ba-p/2077594. Lastly, here are another key reference on enabling child issues planning (non-subtasks) for the basic roadmap - https://support.atlassian.com/jira-software-cloud/docs/enable-child-issue-planning-for-kanban-teams-on-the-roadmap/
Best, Joseph Chung Yin
Jira/JSM Functional Lead, Global Infrastructure Applications Team
Viasat Inc.
You are welcome, if our suggestions helped you, please click on Accept answer when you have a chance.
Best, Joseph