Steam or bethesda has put both the old and new versions into one package on steam which means if you had the old doom3 you also now have the BFG version and if you had the BFG version you now have the old version.
Has gog installers been updated to do something similar?

No. Maybe they will add the old version in the future, maybe they won't.

Hard to know my friend, Doom 3 never was availabe here for what I know, only the BFG Edition, but maybe Bethesda decide to release it here to now that is sold as part of the BFG Edition.

Apparently the reason they won't give GOG customers the original version of Doom is because the original PC release requires a CD key which GOG claims that can't provide for whatever reason, but that doesn't stop them from selling Quake III Arena on this platform which DOES require a CD key that they themselves hand out to customers to play online with that game so I call GOG not willing to go through the hassle of getting the original Doom 3 here and make it DRM-free.

Proof of existence of Quake III CD Keys being distributed by GOG: https://www.gogdb.org/product/1444036272