That's been disabled for ages, so that can't be it. :(
It is this feature https://itch.io/docs/creators/interact
And that feature works for people that do not even have an Itch account. For those it would be the only method of updating them about new content, I guess.
But apparantly, some developers actually join big bundles to send such mails.
Anyway, you do not need to interact with a game to be put on the mailing list. You need to own it. If it is in your library under the purchased items, you are automatically on such a list. I am not sure if I misremember, but it could be that big bundles are supposed to be configured in a way, that you would need to manually add the items from the bundle to your library, for that mailing list thing to trigger. Maybe that changed or maybe the bundle was not configured properly.
Or maybe you did add the games to your library, either manually or by a script. Some older discussions about that issue mention people adding games from big bundles via script. But that's beside the point actually. The mailing list feature is active for single purchases as well, with no way of opting out before you get the first mail.
Thank you!! This has to be it. I thought I was going crazy.
I'm annoyed I can't bulk unsubscribe, especially since it's through Itch, so they know all the lists my email's added to. Someone needs to get Itch's attention with this, it feels like a spam loophole to get around some countries' opt-out-by-default solicitation laws.
Anyway, this helps and breaks it down for me, thanks so much for taking the time to respond.