Manual:Extension support/1.15/ExtDowngrading

Extension.alias.php
The use of $specialPageAliases in Extension.i18n.php does not exist yet, so to support aliases in this file you will need to set the $wgExtensionAliasesFiles array to point to your Extension.i18n.php file containing your $specialPageAliases. Do not create a Extension.alias.php file, there is no need.

$Language::getNamespaceAliases
Language::getNamespaceAliases [1.16] is no longer available. The acceptable fallback is to use the global $wgNamespaceAliases.

OutputPage::getTitle
OutputPage::getTitle [1.16] is no longer available. The acceptable fallback is to use the global $wgTitle. In some cases where your OutputPage is not the global page being displayed this will not be acceptable, at which point you may need to cut functionality or not support pre 1.16 wiki's. This function was often used on $out parameters of hooks, which you can make compatible using the following:

MWHttpRequest and HttpRequest
These classes were introduced in 1.16 and 1.17, but do not exist prior. The recommended fallback is Http::request however at the loss of some functionality (e.g. setting a custom user agent).