Instant alter drop index not supported in 10.4.24

MariaDB [(none)]> \s

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./bin/mysql  Ver 15.1 Distrib 10.4.24-MariaDB, for Linux (x86_64) using readline 5.1

Connection id:		8

Current database:	

Current user:		bdempsey@localhost

SSL:			Cipher in use is DHE-RSA-AES256-GCM-SHA384

Current pager:		stdout

Using outfile:		''

Using delimiter:	;

Server:			MariaDB

Server version:		10.4.24-MariaDB-debug Source distribution

Protocol version:	10

Connection:		Localhost via UNIX socket

Server characterset:	latin1

Db     characterset:	latin1

Client characterset:	utf8

Conn.  characterset:	utf8

UNIX socket:		/home/bdempsey/mariadb/10.2/data/mysql.sock

Uptime:			1 min 8 sec

Threads: 6  Questions: 4  Slow queries: 0  Opens: 17  Flush tables: 1  Open tables: 10  Queries per second avg: 0.058

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MariaDB [foo]> CREATE OR REPLACE TABLE tab (

    ->    a int PRIMARY KEY,

    ->    b varchar(50),

    ->    c varchar(50),

    ->    INDEX b_index (b)

    -> );

Query OK, 0 rows affected (0.053 sec)

MariaDB [foo]> SET SESSION alter_algorithm='INSTANT';

Query OK, 0 rows affected (0.000 sec)

MariaDB [foo]> ALTER TABLE tab DROP INDEX b_index;

ERROR 1846 (0A000): ALGORITHM=INSTANT is not supported. Reason: DROP INDEX. Try ALGORITHM=NOCOPY