Manual:Extension support/1.16/ExtUpgrading

$wgExtensionAliasesFiles
The use of $wgExtensionAliasesFiles is now deprecated. Just use $wgExtensionMessagesFiles to register the Extension.alias.php file.

$wgNamespaceAliases
There is now a new function on Language::getNamespaceAliases introduced to replace usage of $wgNamespaceAliases. This is now critical by 1.18 with the introduction of gendered namespaces aliases which are not stored in $wgNamespaceAliases.

$wgTitle
A local title is now available from some contexts instead of using $wgTitle. If you have a ContextSource object (often $out) then you can retrieve the Title using ContextSource::getTitle. This is the preferred method.

Http::request
HttpRequest is a new class introduced in 1.16 that provides extra functionality to Http::request. For some uses it is recommended. In 1.17 HttpRequest however has been to MWHttpRequest. The recommendation is to use branching logic and use MWHttpRequest when possible. You may want to still support pre 1.16 however because of different calling parameters it will be at the cost of complexity.