Samsung is historically known for pushing out updates to its smartphones at a snail's pace. While the OnePlus 5 had a stable Android Oreo build by December, the Galaxy S8, which had launched nearly 4 months before, did not get the update till mid-February.
Even right now many of Samsung's rivals in India such as OnePlus and Pocophone have already made it clear when exactly Android Pie will be coming to their smartphones, while Samsung has abstained from giving out any details.
However, it would seem that Samsung has started testing the update for the S9 and S9 Plus internally. The S9 Plus was recently spotted on benchmarking platform Gfxbench running on Android 9.0 Pie. The listing shows the S9 Plus as running “Invalid Android Samsung ss Edition.” As per a report by Sammobile, this is just how "test variants of Samsung’s custom skin are labeled in the benchmarks".
In any case, there is no definitive proof that an Android Pie update is coming anytime soon for the S9. If the past is any indicator, the S9 and S9 Plus should receive Android 9.0 Pie at the same time as the S8 and S8 Plus received Oreo.
However, if internal testing of Android 9.0 is currently underway, then there is still some hope for the update to be pushed out before the year closes. We could see the Android 9.0 Beta coming out in a couple of months before any stable build is rolled out.