In the case of a crypto forgeablity argument there will never be any proof. It will always be a false claim. It is a silly idea.
Yes they have, but they have been doing so in the face of circumstantial evidence: DNA, fingerprints, blood, whatever. Or eyewitness evidence: someone saw you go in the building at such and such time (perhaps there is a surveillence video).
If someone has nothing of the sort on you and their entire claim is that you wrote some digital message, I'd think the onus would be on them to prove their extraordinary claim somehow.
* https://articles.59.ca/doku.php?id=pgpfan:off_the_record
* https://articles.59.ca/doku.php?id=pgpfan:repudiability
So I am not disagreeing with what you said, it supports my contention that forgability is a silly cryptographic feature if there is no proof in the first place.