It would be helpful for the development team to see the unused APIs in the sheet and work on it as clients are now more focused on the AI mentor report.

Please help us understand your suggestion.
APIs, such as REST services exposed by the OutSystems platform, are typically designed to be consumed by third-party systems.
AI Mentor Studio works by inspecting source code at design time and, therefore, cannot predict whether APIs will be used in runtime. 
There are other methods to check the usage of APIs at runtime other than using AI Mentor Studio. Can you clarify your request in light of these considerations?

In one of your projects, there are multiple applications handled by different teams. If one team is exposing APIs in different versions (e.g., 0.1, 0.2), and once a new version is created and consumed by other teams, the old version (e.g., 0.1) is no longer used but not deleted. It would be beneficial to receive this information as technical debt in the AI mentor.

I understand, but AI Mentor Studio has no way to know if a REST API is used by third-party systems or not, just by inspecting the code. 

within outsystem application

Anything to show up in the AI mentor should be contributing a technical debt, and to identify one Ai mentor studio matches our code to the code patterns, So I am keen to know that how the AI Mentor studio will come to know that weather a REST API is being used by third-party systems or not.

Maybe a threshold period to be maintained post which the API will be highlighted in the AI Mentor studio as "Not in use from past x days" to which the user may take desired action.

It's a useful idea to know if APIs are being used within Outsystems. Either way, it is possible to query the metadata to obtain this information.