MySQL create index with LOCK=NONE still locks a table
Looks like I have a clue why it doesn't work

Our table have the following constraint:

CONSTRAINT `rpush_notifications_event_id_fk` 
  FOREIGN KEY (`event_id`) REFERENCES `events` (`id`) ON DELETE CASCADE


Next, I found here one interesting thing:
https://blogs.oracle.com/mysqlinnodb/entry/online_alter_table_in_mysql


  Online operation (LOCK=NONE) is not allowed in the following cases:
  
  
  when adding an AUTO_INCREMENT column,
  when the table contains FULLTEXT indexes or a hidden FTS_DOC_ID column, or
  when there are FOREIGN KEY constraints referring to the table, with ON…CASCADE or ON…SET NULL option.
  


so looks like we have a 3rd case