but it did need some kind of content warning
There is no such thing on itch, that I would know of. Not in the sense, that there is systematic support for such a thing.
You either have minor-restricted games or not. And if you do, your account is "nsfw". Wich means to appear in search and browse, you need to have be logged in and specifically activate "Show content marked as adult in search & browse" in settings. I do not know if there is a statement, why an account get's marked as nsfw, but that is what happens. Maybe because the nsfw game will be linked to from your profile.
They could use better terms for all that. Here they call it adult. But internally it seems nsfw.
Anyways, if you want to have some sort of non mandatory content warning, for whatever reason, you need to do that in your game description. Better yet, at the start of the game, if it is something that comes unexpeted. Like flashing lights. Or having a horror gore game that presents like a fluffy goodfeel kid game. Oh, and actual adult games do often sport their own content warnings (or advertisment).
(The nsfw marking is mandatory in those cases where it, well, where it is "nsfw", whatever is nsfw)