This is definitely a decision which requires a long term support, as any solution regarding storage. This is not a technical debt unless you see it as a tempora

This is definitely a decision which requires a long term support, as any solution regarding storage. This is not a technical debt unless you see it as a temporary solution, waiting for a better time to replace it. You highlight the "need to preserve the conversion code for as long as the history exists". This is a point so decisive that I even wonder if Kafka should provide some support to keep that relationship.