As a datapoint I work in 3.8. I will try to move to 3.9 when it seems fruititious - but I probably will not work in it until Seaside, Magma and a few other important pieces are stabilized in it.
My first job in that department would be to give KomHttpServer a 3.9 overhaul of course.
Seaside and Kom work quite nicely in 3.9 today :)
Philippe