I'm an enthusiastic supporter of this change.
I was pretty disappointed to see SE-0084 rejected back in the day. I have often found the lack of trailing commas on expressions frustrating given the circumstances that make them useful for arrays apply to other expression lists too.
The rejection rationale also didn't ring true:
For parameter lists and tuples (the specific topic of the proposal), the trailing terminator of the list is typically placed on the same line as the other elements.
The standard library actually follows a formatting convention (subsequently also adopted by Google's style guide) that absolutely can end up with parameter lists with their trailing terminator on the subsequent line, so all the same good arguments in favor of trailing commas in arrays (like reducing diff noise) also apply. I raise this because a lot of the discussion has been focused on DSLs, but this is an example of it impacting regular code.
So I had for a while been hoping that the subject of SE-84 might get revisited. It had never occurred to me that eliding commas, just like eliding semi-colons, was a far more elegant solution. The subsequent pitch discussion has reinforced this, to the point where, like @nnnnnnnn points out, some of the commas I see are now really bugging me! Thinking about how the standard library would look without them makes me confident this would be a positive change for the language.
I understand that some might feel we've gone beyond that point in the evolution of the language. I really hope not. This kind of change – the adoption of which is entirely discretionary and can be done at the pace of the adopter's choosing – seems like the perfect way to evolve the language and continue to make it better for many new use cases.