(1) you still hold the data, you are still required to comply with the law and cutting off access does not change that one bit.
(2) the period for a response is long enough that once you would receive requests you could handle them in time even if you processed them manually.
(3) you have been - or should have been - aware of all this for a very long time, either you failed at estimating the impact of the law or you do not know what you have or you changed strategies internally recently and now you're not going to be ready in time because you started way too late.
So in all, all you've managed to achieve with this action is to get the spotlight on you, and it is a 100% certainty that at least Instapaper will be solidly violating the GDPR come tomorrow.
If I were in your shoes I would use my designated representative to contact the authorities for guidance after explaining in detail what the problem is before I would let my end users pay the price for my own incompetence.