I don't know if this is connected, but "Free the Sims" was also the name of a very interesting web browser plugin, an ActiveX plugin, created by Don Hopkins.



If they used this plugin on the website at the time of this marketing event, than that would explain why there is no record of it, and the Internet Archive was not able to capture anything substantial from this event. Plugin-based components of web pages, and those plugins, are now long dead -- as the web moved to completely open technologies around 2010, and abandoned the use of proprietary plugins. Microsoft's ActiveX was a technology tied to the Internet Explorer web browser, and to Windows itself -- later deemed dangerous for the computer, as hackers could exploit it.

Edit:

On closer investigation, thesims.com doesn't seem to have even existed before October 1999, and maxis.com was still redirecting simcity.com in April 1999. The "Free the Sims" April Fools marketing event actually took place on the simcity.com website:

https://web.archive.org/web/1999050...000/letter.html

I'm not sure if it included the ActiveX plugin, or if it was simply a web page created, linked from the homepage with a graphic title. Unfortunately it changed too fast for the Wayback Machine to archive it.

Seeing as this was still so early in the public life of the game, I think the ActiveX plugin was created later, and named after this marketing event, possibly. Or they may not be related.