Ok, I think I am understanding the situation now.

I mistakenly thought that there is a traffic island on Yan Ching Street (east side). If there were, then the Y-shape would be outside the junction, and u-turning along the Y would be equivalent to turning right, going forward a bit, u-turning, then waiting for the traffic signal again and turning left again, which is of course allowed.

My intention was to somehow let routers know that they can u-turn along dashed lines. My observation is that, perhaps due to the way routers work, arbitrary u-turn cannot be calculated. The traffic island Y shape convenient allows it. But indeed, here there are no traffic island, so the Y-shape is in the junction, so u-turning along this Y actually remains in the junction, so it should be disallowed.

However, as Kovoschiz pointed out, if there is no u-turn ban of Yan Ching Street eastbound, it might be inappropriate to disallow using the Y for such a u-turn. I vaguely remember a forum discussion about routers evading a nearside turn ban by making a 270 degrees turn. I forgot about the outcome of that discussion, but seeing it is quite similar to this situation here, maybe we can take some inspiration from there.

But still, ultimately, for this specific junction, I still suspect https://www.openstreetmap.org/relation/7239851 doesn't exist irl. Survey still recommended (or Kovoschiz do you happen to know, given you have visited here recently?). If it truly does not exist, then the no-u-turn relation along the Y also should not exist.