Sensitive content / mature content is self identified by the developer. You will have a massive accuracy problem for any filters here.
Itch's nsfw category is culture centric to the United States. That means nude female pixel nipples are nsfw, but gore, horror and slasher games usually are not. Not mandatory, at least.
I am not sure if Steam has a problem here. They have additional maturity settings. Like for the gore, and for mature topics.