How does lock works when delete rows through secondary index in InnoDB?
(Semi-related comments...)

Assuming this is a standard many-to-many mapping table, get rid of id entirely.  Then make one of the other KEYs (either one) the PRIMARY KEY.  More tips here

If the DELETE is typical, then I would make the KEY starting with followable_id the PRIMARY KEY.

This will speed up the inserts and the deletes, thereby tending to avoid deadlocks and "lock waits".