We are not ‘dropping’ messages from smaller instances. @ruud and others did tighten up some things to fend off some attacks but that should not interfere with the federation.
If you could have at least mentioned your instance name we could ask our backend administrators to have a look and we could get back to you with an informed answer.
We will not close registrations, what we will do is show people information on how the fediverse works and give them the option to register on other instances. That will be implemented in the next couple of days.