Water is probably the easiest because it’s already handled in a separate opaque pass. No clue whether it would be performant though.
For other translucency I suspect it’s still pretty far away. Currently it looks like they’re working on Skeletal Mesh support and proper Hardware Raytracing support for the Nanite geo. After that there are a lot of things that need improvements, performance for foliage is still pretty bad, displacement too. Also plenty of bugs; SDF replacement meshes still don’t work with Nanite, anisotropy doesn’t work on Nanite meshes, fallbacks for Nanite geometry collections is broken, probably lots more issues I’m forgetting…
Anyway… I don’t know the answer to your question, my uninformed speculation is that I don’t think they’ll get to it any time soon as it seems like they still have a lot of things left to do and by now most of the low hanging fruit is gone, so tasks just get increasingly difficult.