I'm using the newest version of Xcode, version 13.3.1 (13E500a).
Xcode currently shows a yellow warning sign Update to recommended settings.
When I click on this I get the following:
What I'm wondering:
point: Why does Xcode want to update the Deployment Target to iOS 12? Because it's needed? Or is it just a suggestion, because Xcode thinks I shouldn't support iOS 10 (my current Deployment Target) in 2022? Or is there another reason?
point: As far as I understand this is will allow Xcode to detect a specific kind of compiler problem. But why does Xcode ask me to allow that? Doesn't every Xcode version have many new features, improved error detection, etc. … Why does Xcode ask me in this specific case for allowance to activate the ability to detect this compiler warning?
These recommended settings are just that -- recommendations. You are free to ignore them if you want. It's totally up to you.
To answer your questions:
I think generally the protocol for supported version is the last 2 to 3 major versions (so iOS 12 or iOS 13), but it's totally up to you. Some actually support only up to the last major version. The reason is that iOS users are very good about updating to new versions, and there is data to support that. However, you'll want to figure out if that fits your use-case. I would say generally, there is no reason to support iOS 10 unless a big chunk of your user base is still on that version.
For your second question, compiler warnings can be silenced, treated as errors, or you can keep them as warnings. Again, it's up to you and your use case. It's all configurable, so you can add / remove warnings as you find them useful. This is just Apple's recommendation but you are free to ignore it.
For me personally, I ignore all these recommendations.