Recently on moodle.org/plugins, gallery-lightbox was functioning on moodle 2.5.2.
upon upgrading to moodle 2.6, the gallery-lightbox started having issues (MDLSITE-2260).

The issue i've realised here is that old YUI deprecated code was still functioning properly in 2.5.. It was functioning with deprecated YUI code while all warnings were NOT being shown to developer even in DEBUG DEVELOPER mode. This fails to use moodle developer warning system.

In moodle 2.6, we had upgraded YUI to 3.13 and the deprecated function was finally completely removed. This caused breakage obviously and all that is there in release notes is 'upgraded to yui 3.13.'
It was actually deprecated years ago: https://github.com/yui/yui3/commit/4599ac01a1b5d10be18c377d27f6a357b34220a7

We need to :
1) forward (bubble up with some filtering?) deprecation warnings of third party libs to developers (in developer debug mode).
2) if there is no deprecation system in the third party lib to bubble up (hopefully rare), then we'll need to monitor what third party api we're changing and inform developers with a third party deprecation file/txt/script perhaps.

ps:dobedobedoh has kindly reminded yui guys of deprecation removals in their release notes. https://github.com/yui/yui3/issues/1415