I had a similar question about my short ebooks. The response from Support implied there are multiple levels of projects containing sexual content or depictions of nudity. My understanding—
“Has sensitive content — This project is not suited for minors or the workplace” so it gets the “adult” or “NSFW” filter automatically applied when the box in Metadata is checked. That filter applies a sometimes-hidden tag to the project.
“Show content warning — Show sensitive content warning before accessing game” is the publisher’s choice, but it’s meant for content with a higher likelihood of being triggering.
Relevant tags are unlikely to help a player avoid triggering content, because they’d be hidden under two layers. They might help interested players find what they’re looking for in search. Adding tags is optional except when a site admin or an algorithm applies one from project data.
There’s the regular sensitive/NSFW-marked stuff; the “adult NSFW” stuff that isn’t allowed to accept any payments, tips, or donations; and the stuff not allowed on Itch.io at all.
Everything I make is banned from Stripe payment processing because some of it in obscure places is erotica, but Itch.io (with my agreement) has deemed my “sensitive” work here so far to not be “adult NSFW”. That includes an ebook that mentions sexual acts (that aren’t actually erotic) and nudity (also not erotic).
Okay, so now caveats:
(1) I don’t know how visual media is being categorized.
(2) This doesn’t answer the question about what needs to be filtered.
In the US, currently, school bans are covering everything extremely unevenly. A classical statue has been deemed “porn” in some places, seen as having artistic merit in most places, and in some places allow it only with religious context. A scientifically-based educational textbook carefully made for children goes through the same range of censorship (as “porn”, “obscene”, or “adult”) and acceptance. The arguments about what’s appropriate for legal minors is frequently nonsensical— but this all comes from the same driving force as what Itch.io is responding to.
tl;dr: It’s entirely up to Itch.io Support. No one else really knows where they draw the lines.