As a solution, it is suggested that Bitcoin Core could vomit out a blob containing the final alert to any old protocol version peers that connect, which would greatly improve the likelihood that they continue to get it once propagation of it is gone. This could be left in the codebase for a couple years or until other changes made those old versions p2p incompatible for other reasons. Peter Todd agrees with this idea, stating that it is a good way to document that final alert.