Starting at least with iPhone OS 3.1, possibly earlier, you can choose whether your device auto-joins some WiFi networks. I don’t see this option for my home network, but I caught it on the AT&T WiFi hotspot offered at Starbucks.
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Little features that could make a big difference in iOS and iPadOS
I have a growing list of little features that I hope Apple will add to iOS someday. Since WWDC 2021 was not that day, I’ll share them here.