Just adding on that lack of v2 support caught me my surprise as well, and I’m now scrambling to add v3 support to my project because I expected there would be overlap in support for some time (just as there is in the ansible-galaxy client).
I’m no longer sure where I got that information on, it might have been in the DaWGs working group in March/early April (guessing from the date of https://github.com/ansible-community/antsibull-core/pull/45). Fortunately, for consumers, the v3 API isn’t that different from the v2 API (the complexity mainly comes from supporting both).