Since you've got over 168 hours of playtime and have cleared all but the most recent expansion, the game no longer considers you a novice and has removed you from the chat to make room for new players who need mentors' help and attention more than you do. My suggestion to you would be to reach out to a few of your favorite mentors and ask if they have some FCs or LSes you can join who wouldn't mind answering any remaining questions you may have.

Yes, we know there aren't so many novices still in NN that your removal was necessary, but there has to be a rule to remove people or it would get very cluttered, and you've found the limits SE put in place.