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and if they join the Fediverse, of course they’re going to harvest data from anyone federating with them too.

From the Mastodon blog post about Meta federation:

Will Meta get my data or be able to track me?

Mastodon does not broadcast private data like e-mail or IP address outside of the server your account is hosted on. Our software is built on the reasonable assumption that third party servers cannot be trusted. For example, we cache and reprocess images and videos for you to view, so that the originating server cannot get your IP address, browser name, or time of access. A server you are not signed up with and logged into cannot get your private data or track you across the web. What it can get are your public profile and public posts, which are publicly accessible.

Additionally, the Channel icon in the top right (next to your username) has an option to view only posts from subscribed mags/communities.

Not to the top bar, but here’s a side panel option