When running MySQL/MariaDB with charset utf8mb4, a character is 4 bytes. Hence a key created on a VARCHAR(255) takes 255 characters * 4 bytes/characters = 1020 bytes. However Innodb defaults to a maximum of 767 bytes for an index.
The extension fails to install on Debian Stretch which uses utf8mb4 as the default charset:
Query: CREATE TABLE `abuse_filter_action` ( afa_filter BIGINT unsigned NOT NULL, afa_consequence varchar(255) NOT NULL, afa_parameters TINYBLOB NOT NULL, PRIMARY KEY (afa_filter,afa_consequence), KEY (afa_consequence) ) ENGINE=InnoDB Function: Wikimedia\Rdbms\Database::sourceFile( /workspace/src/extensions/AbuseFilter/abusefilter.tables.sql ) Error: 1071 Specified key was too long; max key length is 767 bytes (localhost:/tmp/quibble-mysql-xo6py6vh/socket)