Only available if:

innodb_file_format = Barracuda
innodb_large_prefix = 1
innodb_file_per_table = 1

Then max key-length = 3072 bytes => 768 characters when using utf8mb4.
Default is to support utf8 chars, installer sets a flag if MySQL isn't compatible and changes the schema back to ascii (behavior as it is now).

Row format for tables objects, assets and documents needs to be ROW_FORMAT = DYNAMIC.