This was a great question about how to mute with the thumb on open chords while keeping the strings ringing out.
This all comes down to correct training of the chord hand.
You need the ability to play open chords with the thumb behind the neck.
Then, AFTER this is trained, you can move the thumb (of the chord playing hand) on top of the neck to mute string 6, and also keep the thumb there for some blues style lead playing.
However, if this is done BEFORE the person has the ability to play well with the thumb behind the neck, the fingers havn't yet learned to stay curled over with correct placement and string become muted.
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