If ssh is using the GNU C Library’s resolv functions, and it isn’t setting RES_TRUSTAD, then glibc will automatically clear the AD bit unless the sysadmin has configured the trustad option in resolv.conf. A competent sysadmin would only do that in cases where MITM is impossible - either a local validating resolver, or DoT/DoH to a remote validating resolver. It is true that ssh has to trust the sysadmin to be competent and not enable that option in cases where MITM is possible, but I think that’s a general assumption for security that the sysadmin is doing the right thing, otherwise there are numerous other possible insecure configurations which might break ssh’s security. To double foolproof it, ssh_config could come with its own trustad setting independent of resolv.conf, so you’d have to set both.