The hashtag search feature first appeared on the service in 2018, but it never worked perfectly. Previously, such a search would include content with a hashtag and other related content that didn't contain a hashtag.
Now, when users search for a specific hashtag on YouTube on desktop and mobile, they will see a special page that contains only videos with that hashtag.
Example of the hashtag #contextualadvertising:
Another way to access search results pages by hashtag is to click any automatically linked hashtag displayed on YouTube. The hashtags themselves are prominently displayed above the video titles.
After clicking on the hashtag, YouTube tells you that the new search results pages will be sorted so that the most viewed videos remain at the top.
One of the interesting elements of the new hashtag pages is the number of videos at the top. Each hashtag page shows how many channels have published videos with that hashtag and how many total videos.
This information can be useful for marketers and content creators because it gives you a chance to look at the hashtag and get an idea of how much total content exists on a topic.