This limitation was for Flow, which now has slightly expanded support.
As far as I know, there are no MessagePoster implementations other than
wikitext and Flow.
Change-Id: Ia0eaeade693f632aa6b9fb9e9f73c41360d5cadf
* Post a message (with subject and body) to a talk page.
*
* @abstract
- * @param {string} subject Subject/topic title; plaintext only (no wikitext or HTML)
+ * @param {string} subject Subject/topic title. The amount of wikitext supported is
+ * implementation-specific. It is recommended to only use basic wikilink syntax for
+ * maximum compatibility.
* @param {string} body Body, as wikitext. Signature code will automatically be added
* by MessagePosters that require one, unless the message already contains the string
* ~~~.