So much legislation consists of "add the following text to USC 1.23: 'blah blah blah'" that I'm surprised there isn't something like this for pending bills.
https://arstechnica.com/tech-policy/2018/11/how-i-changed-th...
I still think there is some merit to the idea. The challenge, however, is to convince legislative bodies to use such a system.
It's a pretty complex task, especially at the state level where there often aren't consistent plaintext publishings, or in some cases even digital versions of the effected code/regulations.
Generally the bills that change legal codes are timed to take effect all in the same near-future date, and so a new “printing” is issued. How often this happens varies, but can be as little as once a year.
In the cases where a law is immediately effected, judges and lawyers are smart and read the news, and services like lexis have alerting. The main legal providers will also “patch” things in themselves in advance of printings, and do change sets and redlines by hand.
Oftentimes bills instruct regulatory agencies to do things, which is a process that includes lots of published notice.
There are pretty much always digital versions, though they may not be freely available to the public or licenses in a way which would permit using them as a basis. (Lexis/Nexis has them, with change history, links to relevant case law, etc., etc., etc.)
Only for already passed legislation, rather than pending, though.