Logs are you best source of truth to determine targets for decommission. Also, it depends on the vertical. If you work for a technology company, it will be a lot harder because there will be a lot attached to resources and Azure and 3rd party cloud resources, APIs, storage, etc. If you are using power platform, there will be studio agents. If youre using Foundry, there will be agent connections. For ownerless, it gets even hairier. There will be addins and other first party services that have created them in the past, etc. You have to look at all details of each one and you will need to script that if you have a lot.