Good point & well made.
It's not been addressed as it's never come up as an issue before, probably at least in part for the reason you mentioned. Currently the only limitations on this are Itch's own, which for mature content are about as comprehensive as "flag it as NSFW & don't do anything illegal".
You're going to have a very mixed audience to consider for this project, so while I'm really tempted to leave this open-ended & instead suggest a solution that would score points in the UX & accessibility criteria, you're right that we should set a baseline.
Continuing with the PEGI standards, then based on what the assessors are expecting to be presented with, aim to mirror PEGI 12 at most. Suitable for teen audiences; violence shouldn't be graphic, mild language is fine, creepy & perilous is fine, probably avoid sex, drug abuse and discrimination to be safe. This is the level that covers games from Overwatch to Night In The Woods & The Sexy Brutale, so it should still allow some decent scope. Depending on what type of fear you're going for, I will point out that something like Among The Sleep is a PEGI 12.
Does this help at all? Would be good to know what you think before I formalise it into the brief.