GitHub now returns 400s if you try to login via an oauth app for device flow if the oauth app in question has not checked the "Enable device flow" option.
I don't know who currently owns the rever oauth app that is used, but if you are blocked by this, you can work around it by creating your own.
Go to https://github.com/settings/applications/new
Give it a name, the callback url can be anything (like http://localhost) since it won't be used.
Then check "Enable Device Flow" and save the app.
Copy the "Client ID" of your new app and paste that into rever/github.xsh around line 44 and replace the listed client id for REVER_CLIENT_ID.
Then the login flow should work to authorize the device