I am sorry that I am only seeing this now, via the activity on GitHub today — we have used the feature (also provided as part of foreman_column_view) to extend the classic / legacy host detail page with various fields.
This was extremely helpful to add custom facts, you can find examples from us here:
Some of these are of course part of the new host details page and now built-in this way, which is great — but there is currently no way (I believe) to add a custom card / a card with self-chosen content (e.g. from custom facts) to the new host details page. @ekohl mentioned in
New host detail page feedback (Foreman 3.0+) - #46 by ekohl
that he agrees that it would be useful to have this functionality built-in such that sites can add custom cards.
I am not sure whether the most likely way to go would be to add this built-in or adapt foreman_column_view to provide only the “extend host details page with a customized part” functionality, updated to the new host details page?