Manual:$wgDeprecationReleaseLimit

Details
Release limitation to wfDeprecated warnings. If set to a release number, development warnings will not be generated for deprecations added in releases after the given limit. Set to false to receive all deprecation notices.

The default value is set to 2 releases before the version of MediaWiki. So 1.17 in 1.19 and 1.18 in 1.20.