I had previously considered adding this type of functionality myself to make syndicating posts (via POSSE) from my own website to sites like Twitter or Mastodon easier. There are a small handful of plugins in the WordPress repository that will add that type of functionality already, but I had eschewed them generally, not wanting yet-another-plugin.
I spent some time trying to track down the plugin that was effecting this change. I couldn’t remember having installed something that would have done this sort of functionality, and I had noticed it only by complete happenstance. I eventually gave up my search halfway through only to later get a message from Matthias Pfefferle that his ActivityPub plugin was the likely culprit. I probably should have guessed as I had literally spent part of that very day looking at the code in his IndieWeb News plugin on GitHub which had code that essentially did the exact same thing, but for a narrower set of results.
The upside of the entire process is that the functionality is now built into a plugin which I’d be using otherwise. As of today’s update, there’s now also a setting for the plugin that will allow one to turn the functionality on or off–I, for one, am definitely keeping it. Of course if you’re looking for the functionality without the extra overhead of the ActivityPub code, I believe you can use Matthias’ WordPress hashtags plugin which does only this.
I’ve never quite liked that Twitter uses @names highlighted within posts. All the additional cruft in Twitter like the “@” and “#” prefixes, while adding useful functionality, have always dramatically decreased the readability and enjoyment of their interface for me. So why not just get rid of them?!
Of course I also remember myself railing against the addition of the symbols @ and # in general text not too long ago, so I’m also now brainstorming and contemplating how one might more quickly (and even in a DRY manner) do this sort of tagging using some other (probably easily accessed, but infrequently used) symbol which could be hidden visually, but which would allow one to add these sorts of tags and the appropriate microformats markup. I suspect there may be some sort of clever CSS I may be able to use too, though it would be better not so that it works easily via syndication and in feed readers with different styling. The goal should be that it would work as plain text from a Micropub client too. With any good luck someone may have thought of it already, otherwise I may be able to hack something simple together to do roughly what I want. The upside would be that simply by writing your post, you could simultaneously be tagging it as well and not need to bother going in and separately adding additional tags!