If you're also on wikitech-l, you may have seen the recent announcement[1] of some work by WMF's Discovery team to improve the search results popped up by the search box in wiki pages. Assuming it continues to receive positive feedback, it's rather likely that the code behind their temporary "action=cirrus-suggest" API module will become the backing engine behind action=opensearch and list=prefixsearch.
This brought to my attention that "list=prefixsearch" has never been particularly well-documented, so Gerrit change 260383[2] was submitted and merged to clarify the issue. Note this isn't a change in behavior, it's a clarification of the existing behavior to avoid future confusion when that behavior is improved.
ApiQueryPrefixSearch was added in Gerrit change 123118[3] to be like action=opensearch but usable as a generator. If your client code is using list=prefixsearch in the expectation that it would work like the similarly-named web UI page Special:PrefixIndex, you should review your usage to make sure that it is actually behaving as you intend. If you are really wanting a list of titles with a prefix rather than a *search* for pages "matching" a prefix in some loosely-defined sense, you should be using list=allpages with the apprefix parameter instead.
[1]: https://lists.wikimedia.org/pipermail/wikitech-l/2015-December/084356.html [2]: https://gerrit.wikimedia.org/r/#/c/260383/ [3]: https://gerrit.wikimedia.org/r/#/c/123118/