Hashtags are the only discovery mechanism on Mastodon, but they're obnoxious to use and only certain small communities use them.
One simple solution I saw someone float the other day: If it's too hard to add a separate "tags" field to ActivityPub posts at this late point, maybe servers should special-format any hashtags at the *end* of the post to be small, gray and unobtrusive? It wouldn't help with char limits, but would be backward compatible… (EDIT: THE REPLIES MAKE THIS LAST IDEA V. FUNNY)