Tl;dr
What
Add a new circuit option to assemblers (and all other machines with the same archetype) allowing to read the current production bonus.Alternatively (or additionally), reading the productivity of the machine, rather than the current bonus.
Why
When making machines that change recipes, it is already possible to read how many items they have crafted, and thus, reverse-engineer if they have hit their production bonus without any wasted bonus on a per-item and per-production line basis.However, if the productivity value changes (which it often does throughout a full Factorio run), it is possible that this detection will cease to work. Therefore, adding a circuit signal for it would be helpful in not having to "hard-code" productivity bonuses.