> doing such tracking will certainly make Writers Guild of America (WGA) arbitrations
> much easier as literally every contribution to a script can be tracked to give screenwriters
> appropriate credit.

Actually that's not true. The only version that matters, are the versions turned in, and those are not the responsibility of the Production Company. Also, there are limited number of credits on screenplays.

Some screenplays have upwards of 30 different writers, only 2 will get any credit (3 if there is a team, and a single writer; 4 if two teams work on the scripts; but that's it). Them's the rules!

Also, some changes to the screenplay would NOT show up in the drafts. e.g. I come up with a story, and sell that story to a production company; they hire a writer to write the screenplay. I get a STORY BY credit, the writer gets WRITTEN BY. If I wrote a book, I would get BASED ON BOOK BY credit.

It is possible to have up to 6 writers on a screenplay, but VERY unlikely.