For years, Twitter itself has been struggling with the problems of hashtags and @names, which while effectual are kludgy and off-putting to new users. In 2014, a Twitter executive seemed to indicate their coming demise:
That executive is no longer at Twitter, and both hashtags and @names remain a constituent part of the service. Hashtags, however, have been rendered less useful by Twitter search. If one tweets “Having fun on vacation!” one’s tweet will surface in searches for “vacation” in the same way as would a tweet that said “Having fun on #vacation!” A hashtag is clickable and can be a badge of sorts for tweets related to live events or conferences or narrow topics, but beyond that utility, it doesn’t alter your tweet’s distribution.
Beyond the evolution of Twitter’s product functionality, hashtags and gratuitous @mentions do seem to be less popular than they were 5 years ago. If they’re out of favor, it’s worth nothing that it’s mostly among certain sets; as with all such conventions, in different communities there are different standards. But it does seem to be the case that they’re gauche among the cool kids these days.
The reason hashtags and the @names of celebrities and such are out of favor among the “cool” is that they signal a possibly thirsty desire for attention, acknowledgment, or participation. Nothing is more anathema to cool people than seeming to want attention, however much they may want it. Visible alterations to a tweet that exist to get @Tim_Cook’s attention or to distribute the tweet to people searching for “#vacation” seem desperate, socially needy.
And there’s also a slightly rude quality to such tweets, as though you’re having dinner in a restaurant while a celebrity (or CEO, etc.) is at a nearby table and you insist on loudly mentioning their name in conversation, glancing in their direction, hoping they’ll invite you over (or at least look back). When people give in to the temptation to seek attention (in general or from the famous), it creates competitive dynamics that are deleterious for the whole community, rewarding self-promoters and shouters.
I’m not actually cool, of course, so this is just my impression. There are also plenty of exceptions, so far as I can tell. But in general, seeming so seek distribution via hashtags or @names of people you don’t know / have no professional or critical or cultural relationship with seems attention-seeking and therefore uncool.