I prefer allowing hashtags over site-wide search. People can use hashtags specifically when they want their post to be associated with a specific search, rather than letting people search for specific words and phrases.

Site-wide search works way better for communities structured the way Reddit or Lemmy are structured, since people can easily run afoul of different moderation policies, and get themselves banned from communities for bad faith interactions. You have no such protections on a microblogging service.