BP Classic is a WordPress plugin by The BuddyPress Community, built to provide backwards compatibility to configs where third party BuddyPress plugin(s) or theme(s) – not ready yet for the BP Rewrites API (introduced in BuddyPress 12.0.0) – are activated.
BP Classic also includes features and templates that are fully deprecated in BuddyPress 12.0.0.
Restored features, once you activated the BP Classic add-on, are:
The BP Legacy widgets (these were migrated as Blocks in BuddyPress 9.0.0).
The BP Default theme.
The BP Legacy navigation globals (buddypress()->bp_nav & buddypress()->bp_options_nav).
More info: https://wordpress.org/plugins/bp-classic/