The Custom Help Collection, available only on-premises, is an excellent idea that is horribly executed. Odds are it was left out of the Online experience because so few installations have taken advantage of the feature.

However, so few installations use this feature because of its horrendous implementation.

Currently we can use javascript to set the navBarHelpOverrideKey, which sets the default product and category for the Help button.
- Recommend adding the product and category as metadata to the Page content type and baking in the override key based on these data elements

Other major issues that would need to be resolved for further adoption of the Custom Help Collection include:
- WYSWYG editor that handles the abnormal link and resource syntax and provides the default layout similar to a Help File layout {search, left navigation, , and content frames}
- Improved farm level publishing/management.. preferably direct editing of the published CAB instead of republishing a new version
- Addition of product and category tags with a help icon to web-parts, enabling context sensitive help