I hear the best way to get the right answer on the internet is to say something untrue and have someone correct you, so I am gonna say that yes, the administrator of your instance can see what you are subscribed to, or at the very least can see what users in their instance are subscribed to. (Please correct me someone if I am wrong)
Things you (depending on lemmy or kbin) upvote, downvote, boost, comment, post, users you follow, and users who follow you can be seen by others (or at least on Kbin we can see them, not sure if the UI on lemmy does the same). Communities you sub to are just seen by yourself however.
I think being able to see upvotes and downvotes is a little too far for comfort. I can confirm that on Lemmy I don't seem to have any way to see what you've voted on.
I have a very poor understanding of what Kbin is - does it allow you to see what I've upvoted/downvoted even though I only use Lemmy? Does it only apply to posts or comments too?
Sorry ignore the upvotes and downvotes part, misspoke regarding those two.
As for what kbin is: Lemmy, but different software. Still uses the same protocol and functions basically the same as lemmy but coded from scratch instead of just using the same software the rest of lemmy does. http://kbin.social, for example)