There are some differences introduced by WSL in Dan Chi Sao such as larger distance and higher aim compared to what I see with your videos that you linked.
I'm quite sure the only change he made was extending the distance. The other points were just bits of instruction on how the drill should have always been done. Aiming low with the vertical palm strike would have always been a mistake.
To be honest the differences between the two seem so great to me that neither could be considered the same drills.
I agree, but I also think the same thing about the entire systems! As I said, without jam-sau the whole system falls apart. Now, maybe others have picked up the pieces and made something else from it and have their reasoning, but it no longer effectively functions in the same way. A completely different type of fighter is developed.
As for jam sao I agree it is removed from SLT from all but WSLVT, YM decided this. WSL brought it back to his own lineage if I dont missunderstand history completely.
I think what happened was that YM suggested using the gaang-sau instead of the jam-sau in the fight situation WSL found himself in, but WSL hadn't learned it yet because it was in the dummy and BJ form. So, YM decided to bring it into SNT to be learned earlier. Others took it upon themselves to replace the jam-sau which had failed, with the "new" technique YM said would have worked, apparently for lack of foresight. They would have to not have understood the system yet and looked at it as just a collection of techniques. So they completely switched one for another based on someone else's fighting experience and not understanding YM's response. Taking jam-sau out would kill the entire system from SNT onward. It is absolutely essential at every stage.
The problem I see with some teachers (not lineage specific) is that they think it better to start out slow by first doing SLT before moving on to the next form. By the time the student gets to touch the dummy and footwork he has already become too stiff in his ways. This is my opinion only.
Are you saying that too much time is spent on SNT before moving on? I agree. It shouldn't take 1 year or more.
But do not be so quick to judge. We cant know that jam sao is removed from these lineages by watching YouTube.
If the section is missing from their SNT, they drop the wrist in DCS, and move directly backward connected at the wrists in SM/TM drills, then it has been removed. They may have another action which they call jam-sau and use it differently, but the jam elbow opposite of taan elbow is gone.