It's up to the instance operators. Lemmy.ml just announced proactive defederation. Some others should follow suit. You as a user can switch instances or operate your own, giving you full control.
Because it's a non-issue since you can create a new account inside the blocked instance if you really have to check some community in there, the defederation can be rolled back and there are multiple similar communities through the different instances (or someone could start their own), so losing one of many doesn't have the same impact as losing the only one.
The point of decentralization is that one group doesn't get to decide the rules you follow. If you disagree with a servers decision to defederate with Meta, then you can switch to another and still take part in the same communities you did before.