https://dev.mysql.com/doc/refman/5.7/en/innodb-restrictions.html

By default, an index key for a single-column index can be up to 767 bytes. The same length limit applies to any index key prefix. See Section 13.1.11, “CREATE INDEX Syntax”. For example, you might hit this limit with a column prefix index of more than 255 characters on a TEXT or VARCHAR column, assuming a UTF-8 character set and the maximum of 3 bytes for each character. When the innodb_large_prefix configuration option is enabled, this length limit is raised to 3072 bytes, for InnoDB tables that use the DYNAMIC and COMPRESSED row formats.

Attempting to use an index prefix length that is greater than the allowed maximum value produces an error. To avoid such errors for replication configurations, avoid setting the innodb_large_prefix option on the master if it cannot also be set on the slaves, and the slaves have unique indexes that could be affected by this limit.

The InnoDB internal maximum key length is 3500 bytes, but MySQL itself restricts this to 3072 bytes. This limit applies to the length of the combined index key in a multi-column index.