Do I have to leave Bluesky to join Eurosky?

No. Migrating to Eurosky doesn't mean leaving Bluesky. You remain fully visible and connected on Bluesky - your posts, followers, and the people you follow all carry over. Unlike switching platforms entirely (the way you might move from X to Bluesky), moving your account is more like moving your data to a different server while your social presence stays exactly the same.