It's easy enough to get the ToS for OnlyFans. It states:
> 5. Do not upload, post, display, or publish Content on OnlyFans that:
>
> a. shows, includes or refers to:
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> i. any individual under 18 years old
>
> ...
>
> vi. violence, rape, lack of consent, hypnosis, intoxication, sexual assault, torture, sadomasochistic abuse or hardcore bondage, **extreme fisting** , or genital mutilation;
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> vii. ...
>
>
Now, this doesn't specify what "extreme" means but instead implies that it is a well-enough understood term that content producers should know.