Manual:$wgBug34832TransitionalRollback/en

Some new features / bug fixes in 1.19 are incompatible with some existing on-wiki interface message configuration. This setting restores the original behavior in 1.19 and 1.20. The setting has been removed with 1.21 and the new code is always used (because this setting is considered a temporary transitional hack).

See bug 34832 for discussion.