CALIFORNIA
How ethical is it for a CHP officer to leave out part of witnesses statements when documented to the REVISED or NARRATIVE SUPPLEMENTAL COLLISION REPORT...
If left out in a supplemental report, were they in the original report?
And, of course, the question would be why they were left out? If they were left out because the officer could not recall the statement or the witness, that's sloppy but not unethical. If they were left out because the officer wanted to blame someone else and that blame would be easier without the statement, THAT would be unethical. Since you post nothing about what the statement would have said or why the officer might have left it out, we can't say.
Also, understand, that depending on the nature of the collision, the statements of witnesses may not even be included - and there is never a requirement to put them in verbatim. If the witness's info is contained in the report (that'd be page 3 in most cases) then you or your insurance company can contact the witness directly for their statement.