OK, found it: https://en.wikipedia.org/wiki/Wikipedia:WikiProject_Encyclop...

"Starting in 2006, much of the still-useful text in the 1911 Encyclopaedia was adapted and absorbed into Wikipedia. Special focus was given to topics that had no equivalent in Wikipedia at the time. The process we used is outlined at the end of this page."

Wikipedia started in 2001. Looks like they absorbed a bunch of out-of-copyright Britannica 1911 content five years later.

There are still 13,000 pages on Wikipedia today that are tagged as deriving from that project: https://en.m.wikipedia.org/wiki/Template:EB1911