The earliest it could have started is when SS7 links and the internet were bridged by dodgy / nefarious owners of said SS7 links. That started to take off arou

The earliest it could have started is when SS7 links and the internet were bridged by dodgy / nefarious owners of said SS7 links. That started to take off around the mid 90's to spam phones with spoofed numbers. I wanted to get the SS7 links terminated but my boss in the wireless industry, tied heavily to SS7 would not let me because they were paying their bill. It would have been one phone call to terminate many of them. I suspect you are probably right about the timeline for swatting as shady VoIP providers started getting popular in the early 2000's and started being used for more than just spoofing text advertisements.