Why isn't getElementById() available on Elements?

The git blame on https://github.com/w3c/dom’s master branch points to:

commit f71d7de304e1ee25573279157dd6ce1c2aa2c4f2
Author: Anne van Kesteren 
AuthorDate: Tue Nov 26 13:53:41 2013 +0000
Commit: Anne van Kesteren 
CommitDate: Tue Nov 26 13:53:41 2013 +0000

Remove getElementById from Element. https://www.w3.org/Bugs/Public/show_bug.cgi?id=23860

and the linked bug describes how jQuery 1.2.5 + 1.2.6 (1.2.x?) are affected:

jQuery 1.2.5 assumes that any node it found in the DOM that has a "getElementById" property is a Document node.