It seems that the OPTION_OBJECTS_FOR_MAPS flag is no longer in use outside of tests. No usage of it in a production code constructor call of SerializerFactory can be found in wikimedia codesearch nor in Github Search.
It was introduced in wmde/WikibaseDataModelSerialization#89 in 2014, but I was unable to find corresponding meaningful usage from the time.
There seems to have been a momentary usage of it in 2015 in I44fa15712d988cdbdcdedbadfa4e7a6f7581a5bb and removed the same day in Ie4bc572accd06d40b3000c522744dd746ff2957f in context of T107865.
It was introduced into the REST API in context of T308438 and then removed again in T322655.
Removing this dead code should make the changes needed for T305660: [LEX] Empty senses/forms lists presentation in dump simpler.