Found the answer in the parent thread, thank you @[email protected]:
That’s a mention, not a tag. A tag is a private description you save about a user. Only apps have this fearure.
It’s a little weird that they took a well established term (in social media context: tag, id by which to mention a user, also known as ‘tagging’) and gave it a wholly different meaning (tag: label).
It’s still in beta and audio appears to not always work when streaming. Though, there’s recent activity on the related issues, so hopefully it gets out of beta before Discord alienates the regular user.
I tested it a few days ago and besides the audio problem it appears to work very well.