Something akin to this is a big part of the products that companies like LexisNexis or Thompson Reuters charge your lawyers a whole lot of money for. Because for a lot of legal questions, you don't just need to know what the law says now, but what it said at the time of the incident in question. And every legislative body around the country (at least at the time I was working in this space a decade ago) publishes legislative changes in completely different and often times not easily parsable forms. Web scraping and PDF scraping was far more common than you'd think. Some state legislatures published changes in more reasonable forms, and some states contract their official legislative compilation to companies like LexisNexis or Thompson Reuters and work to publish an actual change stream with proper standardized references. There's a lot of manual work that goes into making stuff like this work, a lot of lawyers and other legal professionals manually reviewing that the right pieces were removed or added in the right places, matching up references to parts of the law that might have changed between the time the legislation was passed and the time the change was published and figuring out what order everything actually goes in. And the actual text of the law is only part of the equation. Legal decisions from courts can dramatically impact the reading of the law, or effect effectively strike laws from the books (even if the laws are technically still on the books, just not enforceable). So often the right way to represent a diff of two points in time depends on the question you're trying to answer.

You are correct the library of congress and the gpo do this.