It still uses GameBryo for game logic so that is likely moddable. Graphical mods would probably be more complicated.

All of the quests, game logic, AI (including brain dead NOC interactions), voice acting, etc, are exactly the same as in the original and are actually driven by original GameBryo engine. They only rerecorded some lines to add unique voices to NPCs of different races and made some minor gameplay tweaks. The only major changes are graphical - UE5 is used for rendering, all meshes, textures, landscapes and animations are redone. It’s more than a typical remaster like Last of Us, but not exactly a full remake.