This law died a premature death. It was thought to be a good law at first, but it was not conducive to Webboard Mechanics, and there wasn't any solid evidence behind it.
Historical Reference

One forum, degenerate in nature, will always consider another forum, also degenerate in nature, to be barbaric, and vice versa, regardless of which may be more or less degenerate.

Sierra's Law died because its effects were no longer significantly observable: People were trying as hard as they could to ressurect stupid topics, to a point where topics wouldn't even degrade to the point of locking, deletion or absolute stupidity in a clear majority of instances. Sierra's law has been replaced in nature by OctoberFost's Second Law and the xtal-Guapovian Law.

This is the oldest of all laws, and is often "invoked" in stupid topics.
In the proclamation of this law, Sierra first made a divine observation.
Six days later, it was officially declared a law.
This law has had to undergo a revision for logical accuracy.

Any stupid topic may continue to get stupider. The stupider it was to begin with, the faster it may degenerate.

OctoberFost's Corollary: Degeneration may occur until absolute stupidity has been reached, after which point anything can happen.

I do not know how this corollary came into being; It has been taken with this text as an integral part for some years, there instead of the portion on increasing rate of degeneration. I just assume that it was a corruption of Sierra's Law.