The last status from the project: 'Due to another related project, this repo is no longer being actively maintained. (You are welcomed to fork this and continue from here on)'
But it is unfortunate for current Insomnium users that the related project isn't listed or named. There no longer seems to be a Postman alternative works 100% offline, the way a local testing tool should behave.
Source: https://github.com/ArchGPT/insomnium
(Resubmitting this post since the original submission seems missing. Maybe I misclicked somehow)