Active Support's titleize does a bit more than capitalizing each word, but its purpose obviously is not to produce "correct headings for the English language", because that would be a whole and ambitious project in itself. Titleize/titlecase is just a convenience helper that you have to see in the context of the whole suite of inflectors: camelize, underscore, support for acronyms, special treatment of _id suffixes, etc.
To produce headings in English you would probably need to be able to parse the grammar and write something way more sophisticated. Even with a grammar parser, there is not event one set of rules for English. For example, when I looked into this years ago to define
guidelines for the Rails guides I picked one specific set of rules:
When writing headings, capitalize all words except for prepositions, conjunctions, internal articles, and forms of the verb "to be"
But that was may choice and it is not unique for English, so any "titleize" that aims to have those semantics... it is going to be non trivial.
In addition to that, in Catalan or Spanish a title does not even have special case rules, you should be able to capitalize as in standard text. And I bet this is a rabbit hole as soon as you take into account there are more languages than English.
Summary, I don't see this in Elixir core at all.