It looks like Yahoo did indeed end their support of the html library as of 6/8/2017 (according to my error logs). There doesn't appear to be any official announcement of it yet.
Luckily, there is a YQL community library that can be used in place of the official html library with few changes to your codebase. See the htmlstring table in the YQL Console.
Change your YQL query to reference htmltable instead of html and include the community environment in your REST query. For example:
/*/ Old code /*/
var site = "http://www.test.com/foo.html";
var yql = "select * from html where url='" + site + "' AND xpath='//div'";
var resturl = "https://query.yahooapis.com/v1/public/yql?q="
+ encodeURIComponent(yql) + "&format=json";
/*/ New code /*/
var site = "http://www.test.com/foo.html";
var yql = "select * from htmlstring where url='" + site + "' AND xpath='//div'";
var resturl = "https://query.yahooapis.com/v1/public/yql?q="
+ encodeURIComponent(yql) + "&format=json"
+ "&env=store%3A%2F%2Fdatatables.org%2Falltableswithkeys";
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