MySQL RESTRICT and NO ACTION

It is to comply with standard SQL syntax. Like the manual says: (emphasis mine)

NO ACTION: A keyword from standard SQL. In MySQL, equivalent to RESTRICT. The MySQL Server rejects the delete or update operation for the parent table if there is a related foreign key value in the referenced table. Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.

From MySQL Documentation: https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html

Some database systems have deferred checks, and NO ACTION is a deferred check. In MySQL, foreign key constraints are checked immediately, so NO ACTION is the same as RESTRICT.

They are identical in MySQL.

In the SQL 2003 standard there are 5 different referential actions:

CASCADE
RESTRICT
NO ACTION
SET NULL
SET DEFAULT

The difference between NO ACTION and RESTRICT is that according to the standard, NO ACTION is deferred while RESTRICT acts immediately.