It's more than just the checking-for-renewal-info payload, it's also things like how the client should indicate that a certificate has been replaced. Currently the logic is just "Does the directory have a renewalInfo" but in retrospect adding a little more detail about versions to the directory might have been sense. Though at this point, I suspect that there's hope that there won't be any further major revisions.
Really support for ARI and version info for it are just one more thing that should be in some sort of "capabilities" advertised by the server (maybe in the directory metadata), about what CSR hash algorithms are allowed, what key types and lengths are allowed (for both leaf certificates and for account keys), how notBefore/notAfter requests are handled, maybe some rate limit info, etc. (as we have discussed previously).