The argument is that the system as a whole shouldn't be set up in a way to provide (possible, limited) non-repudiation, similar to how postal mail (at least in the US) doesn't provide non-repudiation. It is not something that most people individually would have a say in how it is set up. Publishing expired DKIM keys wouldn't let you claim to the recipient that the message was forged but would make it harder to convince third parties in some contexts that the message is likely to be authentic.
The advantage of deniability is that someone who manages to get one email of yours by any means can't prove to the world that you sent it, which can be an issue with things that are socially considered unfavorable as well as unsympathetic situations like corrupt politicians. There are disadvantages relating to harassment and corruption.
The article doesn't mention it but trusted timestamping during the validity of the DKIM key (plus as always having a copy of the public key) preserves the (limited) non-repudiation. Forwarding during the DKIM key validity could potentially work as a reliable timestamp depending on the details of the forwarding.
The advantage of deniability is that someone who manages to get one email of yours by any means can't prove to the world that you sent it, which can be an issue with things that are socially considered unfavorable as well as unsympathetic situations like corrupt politicians. There are disadvantages relating to harassment and corruption.
The article doesn't mention it but trusted timestamping during the validity of the DKIM key (plus as always having a copy of the public key) preserves the (limited) non-repudiation. Forwarding during the DKIM key validity could potentially work as a reliable timestamp depending on the details of the forwarding.