Lacking an answer from Atlassian I’ll fill people in on what I know:
Jira/Confluence admin UIs allow users to pin their Atlassian products to US, EU or AU regions.
The Connect platform enabled support for app descriptors to provide region specific installation/hosting URLs after the host product changes had been released, leading to a situation where host products were migrated between regions by end-users before apps could add and release support for those new regions.
The Connect integration mechanism for hooking into the region-to-region product data migration is not released as of Nov 2022. This makes it hard for app vendors to handle region-to-region migration of the app data when the host product region is changed.
Forge does not support Connect-on-Forge integration if the Connect app uses Data Residency according to their docs. It’s not clear to me whether Forge apps are deployed to multiple regions or not.
There are some other regions and initatives mentioned on Cloud Roadmap | Atlassian
Edit: there’s also no REST API in Jira/Confluence that will return the region that the host product is pinned to, so trying to resolve the “we couldnt support the region that you pinned your instance to until after you’d migrated your data there” problem is hard. I’ve raised this with Atlassian.