So I just got my stress test. And the current client version is 1.13.2
So they are just continuing onward with the 1.xx.x number format for the classic client.
This will make it easier for Curse to manage the addons regardless of the API version.
Your version of the old clients will still be 1.12.x so, when the developers update their addons to the 1.13.x client and upwards in values, then it should be fairly simple to deal with.