Apparently after doing a little research on the HTML
From the MDN reference:
<isindex>is an obsolete HTML element that puts a text field in a page for querying the document.
<isindex>was providing a single line text input for entering a query string. When sent, the server would return a list of pages matching the query. It supports depended on both the browser and the server to react to the query.
<isindex>is deprecated as of HTML 4.01, because the same behaviour can be achieved with an HTML form.