Manual:$wgNewUserLog/en

Details
Whether to maintain a log of newusers at Special:Log/newusers.

When set to true (default)...
 * users will be able to access Special:Log/newusers to see new user registrations on your wiki. These events will also appear on you Recent Changes page.

When set to false...
 * Special:Log/newusers will report "No matching items in log". This can help to declutter your Recent Changes display, but note the following alternative ways of doing this

Other ways to declutter Recent Changes
If you set any permission restriction on the display of this log file, then the user creations are not written to recent changes (for any permission level). So this permission setting makes the log available to most users, which decluttering recent changes for all users:

Alternatively recent changes is decluttered substantially by using Enhanced recent changes which folds user creations into one line.

replaces Extension:Newuserlog
Replaces, that extension is now obsolete.