Today grdns.cz fixed their 10 domains. The known broken domain count is now 74, and the top 8 list (30 domains total) is now:
10 registry@binero.se 5 hostmaster@dnscluster.nl 4 hostmaster@thosting.cz 3 beheer@metaregistrar.nl 2 hostmaster@pretecno.it 2 hostmaster@kniestdns.nl 2 dns@aerohosting.cz 2 admin@forpsi.net
The remaining 44 problem domains are "singletons", thus likely self-hosted. For some of the domains the problems are only seen with a subset of the nameservers, so they mostly work.
I may post once more on this topic when binero.se fix their DNS. After that, the long tail is noise we can largely ignore.
Ideally the numbers will continue to gradually shrink, but they are low enough to not warrant further individual attention unless perhaps some of the problem nameservers deploy a lot more DNSSEC-enabled domains. With a bit of luck they'll fix their software or firewalls before they do that.