given itch changed their policy, it's entirely possible, we won't get anything here, going forward. at least 1 other vn is no longer available for new purchase, because of the itch policy change(away from allowing adult content). IF digi_b can continue to provide updates here, they likely will continue the same way they've been done up til now, unless and until digi_b says otherwise. steam is a different platform and likely will be handled the way they want things handled. this one, the ball appears to be in itch.io's court, and it's up to them to decide how far they will be pushing their policy.
the only way digi_b can just give previous buyers a steam key is if that was negotiated with steam, prior to it being added. it's highly unlikely they'd accept the idea of "giving away" that many keys, right after steam launch. but, again, that's between digi_b and steam, and whatever they agreed to. and from what i read on the other page, making the game free wouldn't be enough for itch's new policy.
Well, all I know is, how can he give updates to us purchasers who would get them "free" on Itch, because we paid, when others who didn't buy it could also get them? It would need your order number entered first, or something, and I don't know if Itch can even do that!? It's quite a quandary! I wish I could remember which AVN, but there was definitely one that if you paid for it on Itch, you just needed to send the purchase receipt to the dev at a Gmail address, and he emailed a Steam key back. So it can be done.