Keep in mind, this is the opinion of a player, not a publisher/developer.
I suggest you start by making your mind up, who your target audience is.
Browse the tags your game will be about. Then add the "adult" tag and do the same. Is your game in it's "director's cut" at home in one of the two groups, then cater to that group as the primary target. People browsing for games here do have expectations depending how and where they found the game.
Another way of thinking about the topic would be, will your "all ages" version have enough fans for there to be a demand for rule34? A demand you would fulfill yourself. Or would your adult game have enough potential to work as an "all ages" version.
there are still games or works that are R-18 but also have All-Ages versions I haven't seen those in itch.io though
You sure those are all ages versions? 12,13,15,17 are not all ages. Not even 6. An advantage for two versions would be to release a non-18+ version into a market that has restrictions about such games. But that version would probably restriced to be for 15/16/17+ and not be all ages.
I do kinda remember seeing that once in a while, but not often. Games on Itch that have two versions. But more often it would be an adult game that has the option to toggle off the adult bits to make it friendly for streaming the game. Or the demo version would be family friendly, and the paid version not so much. A non adult game where you could install the adult patch? Hmm. Multi-platform releases maybe. But the other version would not be on Itch, but rather on Steam and you would get the adult version/patch here. Something like that I might have seen.
Pragmatically, the adult rating on Itch is kinda unspecific. In the player's settings it is called Show content marked as adult in search & browse. In the metadata it is called Has sensitive content — This project is not suited for minors or the workplace. If you try to leave bits out of the game so you can barely claim it is not mandatory an adult game and not mark it as nsfw, but do heavily promote the nsfw version on the same profile, your game might get delisted or marked as nsfw by staff anyways. "Sensitive content" can mean a lot and need not even be explicit.