I will start working on private communities soon.

Is there not an option in Lemmy to make a community private, or exclude it from /all?

There is no option to hide a community from All at the moment.

Private communities exist, but require to add every member as a mod, unrealistic for anything with more than a few members.

Downvoting a post can be brigading. Voting posts up or down is indeed the primary thing that brigades do.

I did not say that this was an example of brigading. I don’t know that. I pointed out that the views you are sharing imply that you would also be against taking action against brigading.