MARC::Record should proceed blithely on, like Admiral Farragut at the
Battle of Mobile Bay or the Light Brigade at the Battle of Balaclava.
The record can still be serialized as MARCXML, and large records are not
uncommon in the wild. As far as I know, the only way to handle it is to
use MARCXML instead of iso2709.

Ok, but the malformed leader is saved in the MARCXML, so it's displayed in the MARC editor, and cataloguing plugins marc21_leader.pl and unimarc_leader.pl won't work correctly.
Isn't there any workaround for this?

Not to my knowledge.

More than that, Koha is still generating malformed MARC records, and I think it's a problem for an ILS which is MARC-compliant.

It's not ideal, certainly, but Koha is using internally a format (i.e. MARC) that isn't really suitable for the twenty-first century. As far as malformed MARC records go, Koha's offenses are much less egregious than certain other ILSes'.

Regards,

Jared