We’re working on an option in highlighted content to enable custom queries where you can provide the KQL or CAML and specify your own query. This will enable you to search by custom content types.
Thanks for your feedback! Just a note to let you know that we’re building your suggestion now. We plan to increase availability to this feature as we continue improving the tech, and ensuring a smooth experience across Office 365 following the URL changes in SharePoint Online.Brandon Murray commented
It's ridiculous that years after this issue was first encountered by SharePoint Online's early adopters, we still have to request this critical ability!