How to make launch screen work with dark mode but also compatible with ealier iOS version

Aren't system colors enough for you ?

Supporting iOS before 11 is just for a few percent (less than 5% as of August : h ttps://www.macrumors.com/2019/08/08/ios-12-adoption-88-percent/) of the market now. Do you really need this ?

Those users won't be able to upgrade and will just keep your older version.

In addition, remember that launchscreens are incredibly unflexible. So don't try too sophisticated things here.