Manual:$wgPutIPinRC

Prescribes whether MediaWiki should log the IP addresses of its editors in its database's recentchanges table. Although there is no interface in the standard MediaWiki package for retreiving this information, it can be accessed using extensions (most commonly, CheckUser); a database administrator can also run an SQL query to retrieve the IP addresses.

Setting $wgPutIPinRC to false will result in extensions such as CheckUser being unable to retrieve any data which would be stored during such times as it is false (that is, as soon as you set $wgPutIPinRC = false;, IP addresses will not be stored).