Apparently __START__ is no magic word anymore, or rather, it's rendered as __START__ (no effect, like __FOO__), and it's not mentioned in parser.php. I very vaguely recall that it existed in code nine years ago, together with __END__, and the later removal of __END__ is documented. But when was __START__ removed?
Topic on Project:Support desk
It was first added as a kind of i18n hack way back in 2003 in r1610, and ignominiously removed a few months later in r1695. It kept receiving new translations for 4 years before someone finally noticed that it was unused and removed it in r24784. Apparently the purpose of __START__ was to allow comments to precede the content of i18n messages, or some such thing. It never would have worked on content pages, since it was never hooked up to the wikitext parser.
If you have a copy of the MediaWiki git repository on your computer, you can use the search functionalities in the gitk tool to conduct this kind of archaeology. This is how I found all this information out. Very interesting stuff back there in the dim and dusty past...
Done Updated Magic Words documentation page (diff)
Great, no hallucination, but it was in MessagesEn.php. Good riddance, eight or twelve years after the fact.