You determine how far back you want to build deltas for. If you build deltas for the last 3 versions then you can send diffs for those users as well (as long as the dictionary hasn't expired). Or, you could just send the full response just like if dictionaries weren't supported.
Each site can decide what a "good" number of releases to build against based on typical release cycles and user visitation patterns.
Each site can decide what a "good" number of releases to build against based on typical release cycles and user visitation patterns.