Google's documentation states that it will flag any unsafe site but it only specifies phising and malware on their docs so there is no clarity wether it will also flag unsafe sites like:
Does the Google Safe Browsing API flag sites depicting ilicit or explicit content?
Answer: No.
What you are looking for is Google Safe Search. Google Safe Browsing is different from Safe Search. Google safe browsing flags potentially harmful websites based on the pre-defined policies. Explicit content and graphic violence are not included in the Google Safe Browsing policies. But they are included in Google Safe Search.
You can use the Google Cloud Vision API to detect if a site contains explicit content.