There are many reasons that this should be updated for 4.21. This new version introduces better editing features such as editing meshes which should be of interest considering that this plugin imports brush groups as static meshes. This new feature allows the in-engine editing of which faces have what material so that if in Hammer you didn't do texturing because all of your materials are in UE you could then decide where materials go on polys in-engine. https://docs.unrealengine.com/en-us/Studio/Datasmith/HowTo/ModifyStaticMeshGeome...
The old version of this game engine also has a hindering ui bug. On windows the ui does not show drop down menus unless high contrast settings are enabled, (causes white text on white background in hammer in some cases) https://answers.unrealengine.com/questions/591961/drop-down-menus-not-working-in...
I am considering workarounds for the lack of an update do to so many more things being available on the newer engine. Perhaps using the plugin to import materials then opening the project with a new version to make a compatible copy would work for materials. And the perhaps exporting the scene as one fbx file, but I can only imagine the problems that would cause for lightmaps and optimization. It is a good idea to update the plugin because of everything your missing out on and using a newer version would be very difficult or just time consuming. (import map then copy to new engine "oops map has problem" back to step one)