Complex sentence: The DAR adopted a rule excluding African-American artists from the stage at Constitution Hall, built in 1929, following protests by some members over "mixed seating"--blacks and whites seated together at concerts of black artists.
Split into simpler sentences: The DAR adopted a rule excluding African-American artists from the stage at Constitution Hall, which was built in 1929. This decision had followed protests by some members against "mixed seating"--blacks and whites seated together at concerts of black artists.