On the official IRC chat some people tried to help, however this patently is not a common issue. How weird. Who wants to install an OS without being able to verify the image seal? Downloading through HTTPS may be MITM'ed, or the image replaced on the server.
I reported the problem. Somebody also reported it more than a year afterwards.
More than 2 years after my initial report... the ticket remains open. At best the BTS seems neglected.
The FreeBSD maintainers probably agree upon the relative importance of those seals, because they publish them at https://www.freebsd.org/releases/12.1R/signatures.html , however this is neglected in documentation ( https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/b... ). BTW I just noticed that the signing key isn't in the strongset ( https://pgp.cs.uu.nl/mk_path.cgi?FROM=8D12403C2E6CAB086CF64D... )
Everyone involved probably thinks there are more important things to do. I don't complain, as nobody "has to" do anything upon this matter.
My point is that such a way to handle this may reveal that most FreeBSD "hacktivists" just aren't willing to do such menial but necessary onboarding-facilitating job, which would IMHO be useful.
It may be part of the reasons why FreeBSD has way less users than it could and should. In a way it is related to "ease of use", however this "image sealing" is more fundamental than bells, whistles and porcelain.
The ticket: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=222044