Some context: That RFC was inspired by the parallel RFC for constructor promotion:
https://wiki.php.net/rfc/constructor_promotion
With that, it becomes more likely that constructors will be written "vertically," especially when combined with attributes, which are also about to pass:
https://wiki.php.net/rfc/attributes_v2
In that case, having a comma on the last line has all the same advantages as for vertical array definition. When it's inline (the normal horizontal style we're all used to) then the extra comma really doesn't have a benefit.
Disallowing the use of a new PHP feature entirely seems... pointless and counter productive, especially given the use cases it was intended for are about to become a lot more common.
From a process standpoint, PSR-12 is now immutable; it can have errata added, but that's it. We've discussed before that fixed-PSRs are probably a not-good model for a coding standard (esp since PHP 8 is going to have a number of new features that PSR-12 obviously doesn't cover), and this is another example of that.
In the short-term, the best that could be done here is an errata saying "vertical things that allow a trailing comma should allow a trailing comma just like arrays do," but that would by definition be non-binding.
I think at this point the answer is "do nothing until we figure out a new process for living standards rather than just fixed-point standards."
--Larry Garfield