The following test will lock 3 records and their preceding gaps inside InnoDB, even though only the 2 first records match the condition id<=20:
InnoDB cannot be blamed for this, because the SQL layer is not invoking ha_innobase::idx_cond_push(), so prebuilt->idx_cond will be NULL and InnoDB cannot know that the id=30 record does not match the WHERE condition.
To confirm this:
There will be 3 calls to lock_rec_lock() before the DROP TABLE. There should be only 2 calls, with type=LOCK_ORDINARY to lock the keys id=10,id=20 and the gaps preceding them.
Another reason to enable index condition pushdown (ICP) for table scan (or PRIMARY KEY scan) is that if the query would return BLOBs, it would be much more efficient to avoid copying and throwing away the BLOBs of non-matching rows.