Could a 403 error in SmartTube on Android be related to YouTube API changes or Google's security measures?

Yes, absolutely. SmartTube, like other third-party YouTube clients, relies on interacting with YouTube's public and sometimes non-public APIs. Google frequently updates its APIs, security protocols, and terms of service. If SmartTube's developers haven't yet adapted to a recent change (e.g., new authentication requirements, stricter rate limits, or specific API endpoint changes), the app might send requests that are no longer valid or authorized, resulting in a 403 Forbidden error from YouTube's servers. Google might also actively block specific access patterns it deems against its terms of service.