I'm going to have to say [NO] on this one.

Forcing this implementation could be too restrictive for whatever the future may hold, maybe Reddit platform ceases operation, maybe there is a glitch in Donut-Bot, maybe a code gets exploited somewhere - who knows. 

Maybe at some stage there is the idea to reward users that had previous contributions.

People can simply use their governance power to vote NO on proposals they don't support - and if a proposal doesn't get the support because it has a retro-active component, it can be re-proposed without it if the user feels that is necessary.

!tip 1