Last time I checked we had TLS Client Certificates and TLS Certificate Pinning. So yes, we do have mutual auth. Additionally, OAuth2 is not an authentication flow.
I tried out various image labeling APIs, including Google Vision (Safe Search) for exactly this use case (moderation). I was honestly astonished at the pricing of these APIs. Google is somewhere at 1.50€ for 1000 images which is - imo - very expensive. I tried out the default models that come with Tensorflow but well, they are trained on scientific datasets which typically involve species and flowers - no luck there either. Any good tips for pre-trained models that solve this (for tensorflow)?