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)
I'm not in love with the fact that it advertises itself as open source, but when you dig into it, there's a fair number of features that apparently belong to a source proprietary payware fork.
(Not affiliated with them, but am a user. It's not as featureful as Postman, but often good enough.)
I just use an old version of Insomnia from before they added the account requirement and I will use it until the day it stops working.