Added :) I also disabled the "Create Post" button if the community is on a defederated instance even though, technically, you can still post to your instance's local copy (it just won't federate).

Edit: This only works one way. i.e. it can only know if your instance is defederated from the community's. If the community's instance is defederated from yours, there will be no indicator because there's no way to do it without a remote lookup which is both unreliable and inefficient at scale.