The jump from version 8 —> 9 of Confluence means the introduction of what Atlassian calls Platform 7, which made a number of breaking changes to how osgi and apps install works.
Check out these docs for more information:
- https://developer.atlassian.com/platform/marketplace/dc-apps-platform-7/
- Confluence Data Center REST API
- Bitbucket
There is also a number of CDAC threads related to platform 7 generally and Confluence 9 specifically: