It was a feature introduced in the Apollo IOS 16 update that utilized the weather kit api to display weather status from city subreddits.
Here’s an excerpt from Christian Stelig’s notes:
Weather (Yeah!)
Yeah that’s right, that vegetable app isn’t the only one that can do weather! If "Subreddit Weather" is enabled in Apollo, if you navigate to a city’s subreddit (like r/Toronto, r/Perth, r/London, r/Denver, r/SaoPaulo, etc.) at the top of the subreddit Apollo will now show you that city’s weather, as well as the current time at that location. It’s pretty cool, when visiting some subreddits I feel more in tune with what’s going on there now. This uses Apple’s new, really cool WeatherKit framework.