The Message types are:
- FORMAT_PARSE means the message is wikitext and the output is inline-context HTML (although Parser::stripOuterParagraph() doesn't enforce this rigorously)
- FORMAT_BLOCK_PARSE means the message was wikitext and the output is block-context HTML
- FORMAT_PLAIN means the message is plaintext and the output is plaintext (?)
- FORMAT_TEXT means the message is plaintext-with-braces (?) and the output is plaintext (?)
- FORMAT_ESCAPED means the message is plaintext-with-braces (?) and the output is HTML
The page title output is expected to be HTML, albeit "safe" HTML which is stripped of most tags (but does allow <span>, <sup>, <sub> etc through).
Currently OutputPage::getPageTitle() calls Message::text() on the output, I believe primarily because Article titles are *not* supposed to be escaped; ie, an article with the DISPLAYTITLE "H<sub>2</sub>O" should appear with the 2 subscripted, without HTML-escaping the tags. But I believe that is best done by using Message::rawParams() in that one, unusual, case.
Changing ::getPageTitle() to call ::escaped() when given a Message would also make ::plaintextParams() work the way most users "expect", that is, to ensure that the given parameters are escaped. See discussion at https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Wikibase/+/947011.
See T343997: Message should support FORMAT_HTML for an alternative approach.