Does this relatesto the main (first) setup screen for a decoder where the top right values do not indicate Yellow (to indicate a decoder update pending) when the value is changed?

These values are for CS2 pseudo-acceleration/deceleration.

To change the actual decoder settings you would need to use the CV programming , ensuring you know the CVs for your particular decoder.

You are incorrect that it "changes nothing".
The Locomotive setup screen contains aspect of BOTH the decoder characteristics of the Loco AND the CS's configuration for it.

In the case of the ACC/DEC settings - from day one the CS2 has offered a simulation of acceleration and deceleration.
Logically this would most help the older 6080 type decoders with no "build in" capability, but that does not mean it has no value for Locos with decoders that may have this function in built.

With a non zero value, the CS2 itself sends out a ramp of speed commands. The higher the number , the greater the delay with which it does this.
This is most easily demonstrated by setting the values to the max and then , on the Control screen, tap the max value on the speedo and observer the needle slowly makes its way up to that value. similarly for the deceleration.


This is not to say that it might not ALSO write the value to the decoder - I have a feeling you are correct that it may have done this in the past - However I, personally, prefer to have the two values separately configurable.

One other consideration is that not all decoders have two separate CVs for the ACC and DEC or the same eligible value range.
For example, 60760 doco shows a single register(CV) #3 for both Acc and Dec AND its available values are 0-31

Better to NOT make assumptions.
Even for mFX decoders where the CS (theoretically) may know the capabilities of the decoder the user may wish to set up the decoder separately from the CS setting.

Update. Problem solved. Cs2 does change the cv4 braking delay after all. Am using Train controller software. Had to delete locomotive from TC, then add again after I had changed cv value.

Not quite. Tc has option to do advanced speed profile based on decoder characteristics. Once completed Braking compensation can be manually increased. Even increasing braking compensation to 99 percent does not fix locomotive going past blocks. Not using advanced speed profile seems to work for this locomotive. I will seek advice from TC forum.