The b11 is rarely suggested for chords. In my opinion, it is useful if the root chord lacks a third. LydMaj7 (C, F#, G, B), Phryg7 (C, Db, G, Bb), and Locr7 (C, Db, Gb, Bb) could be examples of chords where “add b11” would mean adding the note E to the next octave.