There is no way to read the current value of MASTER_DEMOTE_TO_SLAVE

You can use either of these configurations:

CHANGE MASTER TO    MASTER_DEMOTE_TO_SLAVE = 0, master_Use_Gtid=slave_pos;

CHANGE MASTER TO    MASTER_DEMOTE_TO_SLAVE = 1, master_Use_Gtid=slave_pos;

However, the value saved for MASTER_DEMOTE_TO_SLAVE does not appear in show slave status or in the view information_schema.slave_status.
I do not even know what the default value for MASTER_DEMOTE_TO_SLAVE happens to be because it is not documented.