You could theoretically just loop through every community via "get_communities" and then
com_obj["community_view"]["community"]["hidden"] :: boolean
*I'm writing this on memory, the json structure may be slightly different
and then just subscribe to every community that pops up.
It will likely be slow though, and it's mostly NSFW + !news@lemmy.world that would pop up on c/all if you did. I also *think* lemmygrad as an instance is hidden, so those communities may pop up if they tend to reach c/all.
I'll discuss with the team about making a public list of hidden communities.
We'll take the fact that you haven't noticed much spam as a compliment.
The most recent spam that we have seen is harassment/doxxing campaigns that target specific users. It's not something the average user would notice as the harassment is often DMs or pings on random posts. That, and a ban evader with a recognisable writing style.
As you pointed out, there is no monetary interest on our end. We are just volunteers looking to fix the blindpots in the moderation tools. We have no interest in needlessly censoring speech, and if you ever feel we moderate the instance too harshly, we are open to taking feedback. You can always check the modlog to see our activity. The guidelines for our moderation can be seen here.