Does $wgSpamRegex require any Extensions in MediaWiki version 1.23.5 for it to work?
On the one hand, that page in the Manual says it's "built in" to MediaWiki, but then someone on the Talk page mentioned it's not working and another user recommended installing the SpamRegex Extension, which is obsolete but solved that users problem.
So which is it?
The manual seems up to date. $wgSpamRegex is a core function, and does not require any additional extensions.
(The discussion you are referring to is from 2008, and the MW version mentioned there is 1.13.)