I think we have different definitions of the term "air-gapped". Users still very much connect to it using a network. This just doesn't phone home (or elsewhere).
A truly airgapped Jira-alternative would be somewhat impractical.
Why? I use to manage one on a classified air-gapped network, back before Atlassian was all Cloud Based or 'Data Center' license junk. We had Stash, Jira, Confluence, FishEye, HipChat, everything hosted on a non-internet connected network.
If the local network as a whole can do work, connect to the Kira-alternative for their work, but the network is isolated from the internet - then it counts as airgapped IMO.
A truly airgapped Jira-alternative would be somewhat impractical.