There is an inconsistency in handling comments between direct execution and prepared execution.

Direct execution in 10.3 and 10.11 works as follow:

• a. Executable comment, condition=TRUE

  MariaDB [test]> /*M!50701 MariaDB-10.x does not ignore this */;

  ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'MariaDB-10.x does not ignore this */' at line 1

• b. Executable comment, condition=FALSE

  MariaDB [test]> /*M!200101 MariaDB-10.x ignores this */;

  Query OK, 0 rows affected (0.000 sec)

• c. Non-executable comment

  MariaDB [test]> /* */;

  ERROR: No query specified

So far so good.

Prepared execution (see below) works differently, which is not good.
Moreover, there is an unintentional change in prepared execution (most likely happened in 10.6 by MDEV-16708) - prepared execution in 10.11 for a statement consisting only of a non-executable comment returns OK, which became not consistent with direct execution.

Prepared execution in 10.3:

• a. Executible comment, condition=TRUE

  MariaDB [test]> EXECUTE IMMEDIATE '/*M!50701 MariaDB:10.x does not ignore this*/';

  ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'MariaDB:10.x does not ignore this*/' at line 1

• b. Executable comment, condition=FALSE

  MariaDB [test]> EXECUTE IMMEDIATE '/*M!200101 MariaDB-10.x ignores this */';

  ERROR 1295 (HY000): This command is not supported in the prepared statement protocol yet

• c. Non-executable comment

  MariaDB [test]> EXECUTE IMMEDIATE '/* */';

  ERROR 1295 (HY000): This command is not supported in the prepared statement protocol yet

Notice, (b) and (c) work differently comparing to direct execution.

Prepared execution in 10.11:

• a. Executible comment, condition=TRUE

  MariaDB [test]> EXECUTE IMMEDIATE '/*M!50701 MariaDB-10.x does not ignore this */';

  ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'MariaDB-10.x does not ignore this */' at line 1

• b. Executable comment, condition=FALSE

  MariaDB [test]> EXECUTE IMMEDIATE '/*M!200101 MariaDB-10.x ignores this */';

  Query OK, 0 rows affected (0.001 sec)

• c. Non-executable comment

  MariaDB [test]> EXECUTE IMMEDIATE '/* */';

  Query OK, 0 rows affected (0.001 sec)

Notice:

• (a) works in the same way in both 10.3 and 10.11, in both direct and prepared execution.
• (b) prepared execution (in 10.11) has changed since 10.3. Now it works like direct execution, which should be good.
• (c) prepared execution (in 10.11) has changed since 10.3. Now it returns OK. It used to return an error in 10.3. It should be fixed. The best choice would be to return an empty query error, like direct execution does.