I'm not sure what the current state of things are since I haven't use MySQL recently but this used to be a perfectly valid thing to do.
The issue was that MySQL doesn't use a full int to store enums. If your enum has 8 values, it stores in 1 byte, if it has more than 8, it stores it in 2 bytes. Adding that 9th value thus requires re-writing the entire table. So yes - it can make sense to "reserve space" to avoid a future table re-write.
You also had to be careful to include `ALGORITHM=INPLACE, LOCK=NONE;` in your `ALTER TABLE` statement when changing the enum or it would lock the table and rewrite it.
The issue was that MySQL doesn't use a full int to store enums. If your enum has 8 values, it stores in 1 byte, if it has more than 8, it stores it in 2 bytes. Adding that 9th value thus requires re-writing the entire table. So yes - it can make sense to "reserve space" to avoid a future table re-write.
You also had to be careful to include `ALGORITHM=INPLACE, LOCK=NONE;` in your `ALTER TABLE` statement when changing the enum or it would lock the table and rewrite it.