Alter on huge table consumes all disk space
Make use of the new, better, ways of doing ALTER:

SET SESSION alter_algorithm='INSTANT' ;
ALTER TABLE foo ADD COLUMN ... ;  -- without the AFTER

SET SESSION alter_algorithm='NOCOPY' ;
ALTER TABLE foo ADD INDEX name(col1, col2);


If these cannot be done with the requested algorithm, they will give you an error without doing the ALTER.  From there, you can decide what to do.

A single-column index on a "flag" is rarely useful.  Perhaps you don't need the new index?