@gwynnion This point applies to just about any social media platform. Do you think Twitter/Instagram DMs are not visible to Twitter or Instagram owners? Or that messages you send in Facebook Messenger are invisible to Meta and it's advertising and data collecting machine? Have we so easily forgotten the entire Cambridge Analytica scandal and how large corporations use the data on the social platforms (including private messages) in order to target you with advertising or social campaigns?
It's great that you brought this up in discussion, this just shows the need for everyone to jump ship to a trully private, secure and end-to-end encrypted messenger solution.
There's #Signal which is really easy to set up and use. All you need is a phone number and you're done. #Telegram also encrypts your messages if you use the private chatrooms, and you can also set your phone number to be hidden, and use a user ID instead for searchability. Want even more security and descentralization as well? there is #Matrix and @joinjabber that allow you to pick any server and client of your choosing (Matrix encrypts all your messages by default, on XMPP there is a thing called OMEMO which is server and client (or app) dependent - more details here).
Or you can just create a #PGP key and encrypt the text you want to send on Mastodon like that.
I know it all sounds too complicated for the average folk, on one side, but on the other, if we do not understand technology, it's limitations and it's capabilities, we will never get to use it to full potential.
I put my XMPP address in my profile on this purpose - you can see it if you click on the 3 dots menu while in my profile, then "open original page".