What must I do to acquire a no-cost Weather Underground API?
A capital suggestion. I did just that and besides being bombarded by a host of utterly useless and meaningless ads, I was able to discover that since weather underground has been assimilated by IBM, they have taken on their core ethic. That is: make money above all else. To that end, Weather Underground is no longer providing free APIs.
You're right, no more free keys: https://apicommunity.wunderground.com/weatherapi/topics/weather-underground-api-changes