Are you sure? Based on the official blog post¹, the INSTANT algorithm is new to MySQL 8.0. The manual of MySQL 5.6 indeed doesn't show any "Instant" option in the list of column operations². AFAIK, previous to MySQL 5.6, the fastest algorithm was INPLACE, which copies the data.

¹=https://mysqlserverteam.com/mysql-8-0-innodb-now-supports-in...

²=https://dev.mysql.com/doc/refman/5.6/en/innodb-online-ddl-op...

In the case that you're using MySQL 5.6-compatible Aurora RDS, you need to enable a special flag to get lock-free nullable column additions.