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:

set sql_mode= 'MAXDB';

create table t1 (t timestamp);

show create table t1;

Table	Create Table

t1	CREATE TABLE "t1" (

  "t" datetime DEFAULT NULL

)

drop table t1;

set sql_mode= 'PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,NO_KEY_OPTIONS,NO_TABLE_OPTIONS,NO_FIELD_OPTIONS,NO_AUTO_CREATE_USER';

create table t1 (t timestamp);

show create table t1;

Table	Create Table

t1	CREATE TABLE "t1" (

  "t" timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP

)

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?