Welcome to the community and thanks for your detailed question! 
To get all the checkbox titles from the second st.expander (“Excluded”), you just need to collect the column names that are not in st.session_state.columns. Here’s how you can do it:
Add this line after your for loop (where you create the checkboxes):
excluded_titles = [col for col in df.columns if col not in st.session_state.columns]
st.write("Checkbox titles in 'Excluded':", excluded_titles)
This will display a list of all checkbox titles currently in the “Excluded” expander. For more context and similar UI logic, see the discussion at Streamlit forum: column selection in the dashboard.
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