We used SharePoint Designer 2013 widely for data mining, exploring the content, and editing. Those are the main features that you would need to use SharePoint Designer for.
Further development for SharePoint online would be ideal. However, I'm not sure if it's true that Microsoft will no longer develop SharePoint Designer in the future or not.
SharePoint Designer is not releasing a new version. Version 2013 was the latest version. Now in 2019, we are still using the biggest version. It doesn't work properly anymore. Sometimes we experience issues, but there is no new version.