The most certain answer you’re going to get is that the roadmap shows that there is a subscriptions webhook in the works, and as webhooks are based on Helix endpoints it would mean that there will be a subscriptions endpoint at some point.
Also, with the number of services that are reliant on a subscriptions endpoint it would be a pretty big mistake for Twitch to disrupt those services by killing off v5 before Helix is on par in functionality.