A drop-in is not a plugin, and object-cache.php is one

A drop-in is a file with a fixed name in wp-content that WordPress loads if it exists, and it cannot be activated, deactivated or updated by anything.

$ ls wp-content/*.php
advanced-cache.php    # page cache, loaded before most of core
db.php                # a database layer replacement
object-cache.php      # the persistent object cache
maintenance.php       # the maintenance page

$ wp cache type
Redis Object Cache

object-cache.php is usually copied there by a plugin’s activation and left behind when the plugin is deleted, at which point the site is using a cache backend whose plugin no longer exists. That produces failures nobody can trace to a cause, because the file is not in any plugin listing. Checking for stray drop-ins on any inherited site is a two-second command and is worth it.