> we largely abandoned JIRA and years later the instance got turned off and deleted
Sorry to be nitpicky, but why did you abandon a tool that contained a lot of valuable knowledge? That's not the fault of GH nor JIRA, that's your fault. At least you'd back up descriptions + comments from these JIRA sources.
You can trivially export your data from JIRA. If the parent experienced a situation where valuable information was lost because the instance was deactivated, that's not JIRA's fault.
That isn't true though. It's very easy to export your data from JIRA. From your board, go to the List tab, filter the items to whatever you want, and then click ... and you can export the data in various different formats. Exporting as XML dumps everything.