I would like to remove "support" for the obsolete isindex element from Blink. I put support in quotes because what we currently do with the tag is strange, and doesn't seem to actually support it's usage in the wild.
Currently, blink parses the isindex element by magically inserting a form and input element, along with some explanatory text (see HTMLTreeBuilder::processIsindexStartTagForInBody). This ends up looking like the "This is a searchable index. ..." at the top of , for example.
Typing something into that field and submitting the form reloads the page with the search query appended as a query string. In my testing, this has correctly searched for something on exactly 0 pages.
Given that the element is interpreted in a way that injects an unexpected form into a document, there is some non-trivial risk of a content injection attack making use of this element that probably isn't high on folks' lists of elements to filter
I don't have UseCounter data (and it's nontrivial to get, since the replacement happens somewhere where we don't currently have easy access to the counter mechanism). Grepping for the text through Google's search index reveals minimal usage: 756 results total on 341 unique domains (this includes pages like  that merely mention the tag; the tool's syntax is strange, and I don't know how to filter those out).
Firefox supports isindex in exactly the same way that Blink does. Pages that rely on this behavior will break. So far I haven't found any.