I think we might be talking about different things, so just in case I'm going to clarify, sorry if it's too much! I figured if I am confused, OP or other readers might be as well :)
When the OP mentioned wanting mods to come to iOS, to me that means being able to download and install mods like you can on PC/Android. It's technically possible to, say,
edit your save to give yourself extra money or whatever, but to me that isn't being able to use mods. Likewise, while if you jailbreak it's possible to use some XNB mods of the old variety that just overwrite the game files directly (much like Switch modding), the vast majority of non-broken mods don't work that way, so in most cases you'd also need to
create them.
(This is in addition to
all the problems inherent in XNB modding in general, too :P)
Rather than being "open a game file and edit it" replacers for example, almost all up-to-date basic texture replacers require
SMAPI and have much more advanced functionality and compatibility than old direct replacers. SMAPI has a
port for Android, but
not for iOS. This means that most Stardew Valley mods you find on Nexus work just fine for Android unless otherwise noted, but not for iOS, even if you jailbreak.
Someone else would be able to comment in more detail about the many challenges of trying to develop SMAPI for iOS, but in short it's not just a matter of file structure because SMAPI modding involves a lot more under the hood than just overwriting a file in the content folder. (Android is hard for people to keep up with already, and it's got a lot of rewrites in it handling the differences between the two codebases, too.) So that's what the main parts of my comments were about -- that it's not really down to the game devs, that it's really unlikely "normal" modding will come any time soon but technically possible (? maybe, if jailbroken?), and that very simple overwrite modding is possible with a jailbreak but that no one is going to be able to recommend/help with that directly.
Hopefully that makes sense, I feel like I have been rambling a bit :D