Deleting old files in /var/lib/postgresql/9.x/main after changing postgresql data_directory
There shouldn't be any references as all the scripts/utils/etc use the data directory environment variable.

the only place I would look is in the new configuration file - take a look you don't have any includes that point to the old folder (not sure at what version they added this option.)

BTW, it's always a good practice to rename (mv dir dir_bck) the folder for a week to see you really don't have anything trying to access there.
(you shouldn't anyway as it's stale data...)

Regards,
 - Jony