Sorry for the delay in getting back to you. I have some ideas but I may need some additional information around your requirements here. Octopus does a good job at cleaning up old data with any significant size, such as packages, releases and deployments. This is covered in the Retention Policies documentation I previously linked.
It sounds like you would like to see a list of all projects and their deployment history. This sounds like a job for the Auditing feature. The Audit log allows you to search for any activity/changes in Octopus and show you when it happened, what was changed, and who made the change. You are able to apply any number of filters to the search, such as project, target, user, channel, etc.
So if you are using the Audit feature to search for all deployments for a specific project, you can manually cross-reference that with your retention configuration to ensure that your targets/server are currently keeping the desired amount of deployments/packages, or that the Octopus server is not holding onto any releases/packages.
The swagger UI is handy for seeing a list of the CURL commands you can run against the server and getting you started with the API, though it’s not the best for comparing data, which is why we have developed our auditing tool to do this function well.
I may be misunderstanding your exact requirements, if my above ideas are not sufficient, I’ll need to get some more details around your specific requirements and concerns you may have around the Octopus retention functionality.
Let me know what you think.