As discussed in the C3 runtime blog post the new runtime relies on modern browser features. The minimum support could have been iOS 11, but due to severe bugs in Safari that are fixed in iOS 12, we decided to set the minimum version to iOS 12 by default instead. According to Apple's data, this covers 80% of all devices, and 83% of devices released in the last 4 years, so it already covers the vast majority of devices and as ever that is always increasing. We don't want to increase the minimum requirements any further, but we may do so in the long term if supporting older versions is either infeasible, or they have reduced to negligible market share.
Off the top of my head I think the iOS 11 bugs prevented Opus audio playback and Pathfinding from working. If you test carefully and everything works OK for your project, you may be able to release for iOS 11 as well, but I cannot make any guarantees.