When/how does removing an index cause a table to lock in MySQL?

Question

Dropping an index in MySQL 8 is INPLACE, not INSTANT, per https://dev.mysql.com/doc/refman/8.0/en/innodb-online-ddl-operations.html. As I understand it, INPLACE means that there is potentially some table locking involved. We are looking to drop some indexes that are no longer needed from an 80M row table. Testing on a staging copy of the db showed the indexes being dropped in a few hundred milliseconds, so the operation is potentially very fast. Is there any chance that it would cause substantial locking (more than a few seconds) on a production table that has many reads/second and about 1 write/second on average? If so, what conditions should we be looking out for that could cause that? (To reiterate, these indexes are not used by any queries.)