CKEditor5 readiness issue to tackle all the thing to make this module work with CKEditor5.
Enable CKEditor5 and select it as editor. Use this module in the same text format & enable HubSpot forms filter.
It gives the status message:
CKEditor 5 only works with HTML-based text formats. The... ... filter implies this text format is not HTML anymore.
Update filter code to make it compatible with CKEditor5.