With the release of APIv4 in ATCv6, should we simultaneously deprecate APIv2 and APIv3? I think so, that'll mean we can remove them in ATCv7, whereupon the stable API 4.0 will have existed for a full major rev, and APIv5 will ostensibly be released (if not sooner, since we could do that e.g. in a 6.1).
If so, we should also discuss what that will mean materially. With endpoints that disappear between API versions we have them return warning-level alerts that indicate they won't be available on upgrade, but for APIv1 as a whole we didn't issue any kind of formal notice afaik, not even a changelog entry. I think the right answer is somewhere between these - a changelog entry and notices on the APIv2 and APIv3 reference sections of the documentation. I don't think it's necessary to mention on each endpoint that the entire API version is deprecated, either in the documentation or in the API through Alerts.
