Many distros (including Fedora 31) provide assimp as a package. If doomsday is built with embedded assimp, resulting RPM would conflict with system assimp package, so I build RPM for Fedora 31 using system provided assimp. That means that package should explicitly require assimp when installed. I modified doomsday/cmake/Packaging.cmake, added the following block after line 20:
if (DENG_ASSIMP_EMBEDDED STREQUAL OFF)
string (APPEND CPACK_RPM_PACKAGE_REQUIRES ", assimp")
This ensures that assimp package will by automatically installed by dnf when you try to install doomsday RPM.
CPACK_DEBIAN_PACKAGE_DEPENDS should be modified in similar way, I just don't have debian to check proper name for assimp.