It affects the data encrypted by GPG until that "new and improved" version, as long as the keys have "older" format. Until recently the GPG was able to use such "older format" key for both encryption and decryption. Now it can't do both. And that was even not necessarily known by the users, that they used something "older": you saw the shorter fingerprint but otherwise everything worked.
GPG even doesn't inform the users in the runtime that it silently removes user's "old format" keys from the set of keys they had. They just "dissaper."
The people who use the PGP the longest are the ones most inconvenienced. The old data, supposed to be backups, can't be read by the new version.
We must maintain perfect backward compatibility back to the beginning of time! We must have pristine clean and cruft-free code to help ensure mistakes are easy to catch!
Sounds like the age old, More Taste! Less Filling! Backward compatibility for insecure algorithms is exactly the code which should be jettisoned. We have VMs which run Amiga and DOS, if you're terrified of not being able to decrypt then grab a cup of coffee and get to re-encrypting with a key that doesn't inline MD5!
You did get the part where this is free software, and you are free to fork it if you wish?
I tend to think it's the users responsibility, by choosing to use the package, to actually understand how it works. The maintainer owes you nothing. It's stated right there in the license.
P.S. The scare quotes do not help your argument. The old keys are technically weak. That happens with crypto from time to time. If you can't plan for that, might as well keep it cleartext.
While I agree that support needs to be dropped eventually. Printing a warning and only allowing decryption with weak keys would have been a better option IMO.
Keep that around for a year or two, then drop support fully.
GPG even doesn't inform the users in the runtime that it silently removes user's "old format" keys from the set of keys they had. They just "dissaper."
The people who use the PGP the longest are the ones most inconvenienced. The old data, supposed to be backups, can't be read by the new version.