Linking non-universal library to support older Macs into universal build

Have an i386 / x86_64 built library that can't be re-built as universal, but it's needed to support (for some time to come) Intel boxes back to macOS 10.12 Sierra. Provided I don't use the library on Apple Silicon, why can't I include the library (and its header) for use in the x86-64 slice of the app for Intel or Rossetta 2? Xcode immediately identifies the library as a problem when compiling for universal, but I can't (or there doesn't seem to be a method to) incorporate that library anyway. If there's no work-around, I'd like to hear the technical reason the lib can't be included in the universal build. The prohibition may keep me from accommodating Apple Silicon for quite a long time.

This is an eSellerate serial number validation lib - no longer supported - and it's needed for older customers (guaranteed to be running on Intel boxes) who purchased our application outside the App Store. These customers still upgrade their software and hardware appropriately and as long as they keep their Intel box, I'd like to support them (except demo copies, all recent customers must originate from App Store). The algorithm embedded in the lib is proprietary, so I can't reproduce it.
Set the VALIDATE_WORKSPACE build setting to No explicitly for the target. In other words, don't let that setting inherit from a different level of your build settings. With that set, the build system won't validate that all of your dependencies support Apple Silicon.

The client code that calls into the library needs to be guarded with the appropriate target macros, otherwise you will get linker errors for unfound symbols in the Apple Silicon build.
Linking non-universal library to support older Macs into universal build
 
 
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