The CheckUser extension stores information about each change to the wiki for a fixed period of time (by default, three months). This information includes the following database fields for each action to the wiki:
cu_changes.cuc_user – account ID of the user performing an action; this would be used for self-lookups; it's indexed [(cuc_user,cuc_ip,cuc_timestamp)] cu_changes.cuc_ip – IP address [IPv4 and IPv6] cu_changes.cuc_xff – XFF data cu_changes.cuc_agent – User-Agent data
By default, MediaWiki core also stores private information in the recentchanges table:
recentchanges.rc_ip – IP address
In the interest of freedom of information and enhancing account security, it should be possible for users to see the private data stored about themselves at any time.
The implementation of this idea will be done though an extension (Extension:AccountInfo).
For more details see Retained account data self-discovery RfC.