There’s something on their page about it being EOL on Jan 3, 2019. There appears to be some sort of replacement RSS feed, but you need to get separate access keys.
https://developer.yahoo.com/weather/
Important EOL Notice: As of Thursday, Jan. 3, 2019, the weather.yahooapis.com and query.yahooapis.com for Yahoo Weather API will be retired.
To continue using our free Yahoo Weather APIs, use https://weather-ydn-yql.media.yahoo.com/forecastrss. Contact yahoo-weather-ydn-api@oath.com for credentials to >onboard to this free Yahoo Weather API service.