When compiled with -* +collada, `assimp listext` subsequently confirms that there is no support for *.dae. Meanwhile, when I build assimp 5.4.3 from source with what looks like the same options in ccmake, I do get *.dae support.
Created attachment 918265 [details] emerge --info assimp
Caused by typo. Uses "use" instead of "usex". Can be mitigated by passing MYCMAKEARGS="-DASSIMP_BUILD_COLLADA_IMPORTER=yes -DASSIMP_BUILD_COLLADA_EXPORTER=yes" in env for the time being.