create table t1 (a int, b int, key a1(a)) engine=innodb;
insert into t1 values (1,1),(2,2),(3,3);
alter table t1 alter index a1 ignored, algorithm=instant;
show create table t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  KEY `a1` (`a`) IGNORED
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_uca1400_ai_ci
alter table t1 alter index a1 not ignored, algorithm=instant;
show create table t1;
Table	Create Table
t1	CREATE TABLE `t1` (
  `a` int(11) DEFAULT NULL,
  `b` int(11) DEFAULT NULL,
  KEY `a1` (`a`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_uca1400_ai_ci
drop table t1;