SQL_MODE MAXDB is documented as an equivalent of PIPES_AS_CONCAT, ANSI_QUOTES, IGNORE_SPACE, MAXDB, NO_KEY_OPTIONS, NO_TABLE_OPTIONS, NO_FIELD_OPTIONS, NO_AUTO_CREATE_USER. It's a recursive definition in itself, the only meaningful way to read it is that it's an equivalent of the combination of the other listed modes; but it is not. At least one extra effect is that MAXDB mode makes timestamp be converted into datetime, while the combination of the listed modes doesn't:
It was the same in MySQL up to an including 5.6; in 5.7 MAXDB mode was deprecated, and a warning about implicit conversion was returned. in 8.0, the mode is removed.
At least the documentation needs to be updated, but maybe MariaDB doesn't need the mode either?