>iOS 10 is the Maximum Deployment Target
No...not how it works. Deployment target (aka 'Deployment OS') is the minimum iOS your app promises to support. Base SDK is maximum, which is always controlled by the tools/Xcode and whatever the latest SDKs/OS support it comes with. If you want to support an SDK/OS max version (base SDK) lower than the current release, you'll need to use an older version Xcode.
>I use XCode Version 10.
In that example, you can't get there from here. In that example, iOS 12.x support is mandated, regardless of deployment target.
• See the xcode wiki for which version has base sdk of iOS 10.x. Cross reference that info at: See supported versions
• If you find a combination that is still allowed for builds/uploads to the store, go to dev Downloads/see more downloads and find it there.
My advice is to avoid all that trouble and do whatever is necessary to be able to support current iOS 12.x, and iOS 11.x, and their larger than iOS 10.x user community, instead.
It might seem difficult now, but it will be worthwhile upfront work that will benefit you and your users for some time to come.
Good luck.