All we know right now is the typical FUD* here: https://developer.apple.com/ios/submit/
"Starting March 2019, all iOS apps submitted to the App Store will need to be built with the iOS 12.1 SDK or later."
Parsed as 'must be built w/an Xcode that can handle 12.1' - Taken to mean Xcode 10.1 or higher. Right now, the minimum for Xcode 10.x is macOS High Sierra/10.13.6.
However... ASG routinely allows older Xcode/IOS. See: Supported Xcode and Application Loader versions ...for what ASG currently accepts for minimum Xcode & iOS.
As long as you can basically push apps w/tools that ASG supports, you can stay where you are, of course...we don't know when the day will come that you as an individual will be pushed to move up, just keep in mind that there have been examples in the past, such as w/Metal, where even tho older tools were still generally allowed by ASG, certain type apps required a current macOS.
*I say FUD, because we've seen similar admonitions in the past, only to find that ASG -still- allows earlier Xcodes/SDKs.