I get to correct the esteemed EDCC. (doesn't happen very often )
Classic support is included in the default OS X install, up to OS X 10.5 (Leopard) when Apple removed that support.
Prior to 10.5, there is a System preference pane, which in later versions does not appear, unless you have an OS 9 System Folder that the system can see. That does mean that you need to have OS 0 installed, or a valid OS 9 System Folder can simply be available to your Mac. That pref pane allows you to control, and do limited maintenance of the OS 9 system that may be installed.
Again, Classic support was removed in 10.5 (Leopard), so if you need Classic, you will need to downgrade to an older version of OS X.
Serendipity is a lucky guess !