[grey-area]

None of this is related to continuing to use the Twitter API or our commitment to enabling developers to build on our platform

It is directly related.

Twitter desperately needs to decide whether you are a platform or a closed business. If you're a platform your job should be encouraging any and all innovation on there, and have clear pricing tiers which do not result in sudden jumps from free to large charges or access cut off and directly reflect the cost to you of serving the API. You should never be in a situation where you're emailing to cut off access (and yes this notice is shutting it down).

Successful products built on twitter should be celebrated, not forced off the platform.