With very little fanfare, there has been a change to the hashtag-following experience in SharePoint Online. Up until very recently, when you clicked the numbered link above tags on your Newsfeed page, you were directed to the Newsfeed Settings view of your profile, as shown in the following figure:
Now, when you click that same link, you will see something slightly different:
This look is consistent with the pages for followed documents, sites, and (to a lesser extent) people. If you do not want to follow a particular hashtag, just click the corresponding Stop following link. Interested in following some other hashtag? Just click the follow link that is to the right of the profile picture and specify a #tag.
Generally, this is a good move by Microsoft in that it helps standardize the overall social experience in SharePoint Online.