The first iOS version of Stardew Valley came out when iOS 12 was the current OS so unless that game version also happened to work on iOS 11 or earlier (it may have), there's no way to get a working version of Stardew for those OSes. iOS 7 and 8 are certainly out. And that was game version 1.3.28, so missing all the content and feature updates from v1.4 through v1.6 which are pretty big.
I checked all my older iOS devices, some as far back as iOS 7.1.2 but I don't have Stardew installed on any of them, I just didn't buy it on mobile until much later. If you find someone who made a local backup of the older game package, you might be able to sideload it using an older version of iTunes but even then licensing is always a sore point with some things refusing to run thanks to license purchase date inconsistencies.