Introducing Simon 5!
Simon version 5 is now available. It is a massive update, introducing a much-requested feature: syncing the Simon data between multiple Macs, plus a Simon Status service, a Link Checker filter, improved Find filter, iMessage and Speak Error notifiers, a new app icon, and much more.
Important: please note that Simon 5 requires a minimum of macOS 10.12 (Sierra), and is a paid upgrade from Simon 4. Purchasers since September 1, 2020 automatically get a version 5 license (that also works in previous versions).
Re: Unix diff for changes in web pages
Yes, you could actually use a unix diff if you wish, via a Script-based filter. Version 3's powerful new filters features enables lots more analysis of service output.
Of course, the test will also display the difference between the current and previous checks in the Changes log, if you don't need the detailed diff.