Hi,
I would need to some way get access to the issues development data from the Jira plugin written using Forge. I know that data is available through the rest/dev-status endpoint when using basic auth but that endpoint doesn’t work through the plugin. Is there actually any way to get that development data from the issue to the plugin 
Welcome to the Atlassian developer community @JoonasVenlinen,
No, unfortunately, there are no public APIs for consuming the development data that work with Forge. Overall, the “plans” for consuming that remain in the same state as when discussed 2 years ago: