And long before v5, there was v3, which v5 was almost entirely based on with the main exception being that v5 shifted the focus to using an id in request paths/querystring params rather than a username. The v3 docs for that subscriptions endpoint (which is the exact same url BTW as the request in v5) goes back to feb 2013, and if there was documentation prior to that then that was before my time here, either way that created_at field has existed prior to streaks as George has said.
Again, just because it’s not the data you want, doesn’t mean it is incorrect. Perhaps the docs could do more to explain the purpose of that field so as to cause less confusion (even some of the most well known dev companies on Twitch have made mistakes due to misunderstanding that endpoint, so that does show the docs leave something to be desired, but thankfully that’s being worked on and there’s even a dedicated Docs person at Twitch now!).
As you can already see from the changes going forwards from v5 to Helix Twitch is already making great efforts to remove both redundant data and data with minimal usefulness (while the notion of created_at being a bug is wrong, its usefulness is certainly limited and devs have put forward better suggestions that would be more useful in Helix).
As has been said, you’ll have to wait for Helix. Feel free to submit your suggestion to Twitch yourself though if you like: Twitch API Feature Requests (please use twitch.uservoice.com)