How can I set data usage limits on my iPhone 16?

Here is a link from Apple, which tells you ways you can limit data usage, but there has never been a feature to set a limit to data usage on iPhone --> Use Low Data Mode on your iPhone and iPad - Apple Support

Since this is a user to user only forum, you can provide a feature request to Apple here --> Feedback - iPhone - Apple

It is becoming increasingly less common to have metered data plans. It has now been years since I had to even worry about my data consumption. If it is a concern to you, you might want to investigate plans with your carrier, which don't limit data usage.

I understand you find it odd, but again, this is a user to user only forum. We don't work for Apple and we aren't informed on why Apple does anything the way do. All we can do is give you what the available options are and to let you know how you can pass feedback along to Apple if you want to. Your phone isn't limiting your carrier nor your plan choice. That is entirely up to you.