There's an Archive Team project for the government, although it's just trying to get as much of a snapshot as possible: https://wiki.archiveteam.org/index.php/US_Government Detecting changes across all pages in real time is going to be difficult to accomplish.
You can use a combination of the website capture data available in the Internet Archive along with what the Internet Archive and ArchiveTeam crawl to analyze and track when page status changes (both content and http codes, 200->4xx for example). Look for the diffs across the .govs of interest. Crawling continues.
(no affiliation, friendly reminder to donate to the archive)
