It will likely just be via badges tag, although the subscriber tag could work as an additional data point, so instead of just 0 or 1, you could have the number of months or the level of loyalty badge in it.
Yeah, it’d be interesting if they also passed the total months in the subscriber tag, they could have the badge in the api end point: "https://api.twitch.tv/kraken/chat/CHANNEL/badges and we could pull from there based on the subscriber tag (if they pass total months).
But if they pass the ID directly through the badges tag, that’d much easier (for us and for them).
Edit: Since user will also most likely be able to opt in/out of showing this badge, it’ll probably be via the badges icon. Or will it replace the sub badge icon we see now?
Double Edit: It seems that the loyalty badge will replace the existing sub badge today (which in the future, is the “1 month loyalty badge”. So I am supposing that it’ll work the same way for today’s Sub Badge, twitch will just return the appropriate badge.