Comment by Tabular-Iceberg

Cinepak was one such codec and that could be arbitrarily seeked and copied just fine, even in the early 90s, if the player was competently implemented. It's just a matter of computing from the nearest keyframe.

What really happened was that the feature was first paywalled as QuickTime Pro, then removed altogether, in typical enshittification fashion. It had nothing to do with the technical limitations of any of the codecs.