I think the blog post is pretty clear about that:
Before reaching the final state, State 4, an app must not store any data outside of the Atlassian cloud (either through migration or by design). In addition, an app must migrate any remote-hosted compute in the Atlassian cloud.)
So yes, if you want your app to reach State 4, everything will have to be on Forge. However, in the blog post it also says this:
Although strongly recommended, State 3 and State 4 are not hard requirements, leaving room for exceptional cases where apps cannot move away from “off-Atlassian” compute and storage. Examples of exceptional cases include:
- Apps that are by nature integrations between the Atlassian platform and external applications.
- Apps with extremely heavy compute and storage requirements that exceed Forge quotas.
- Apps that require massive amount of data for machine learning or reporting purposes.