What exactly is the plan? To mandate some client side checking so that transfer encryption backdoors won't be needed?

At what level would you need to do that? E.g. for iOS and then iOS need to comply with every app store app having it or else they can't operate in the EU? Is that the plan?

Every chat application must comply by law and companies which make them without government permission will be made criminals. It's similar as the UK Online Safety Act: 4chan staff will be arrested if they ever touch the UK soil.