PostgreSQL what happens if TimeZone is not set?
It seems like PostgreSQL 10 still


Looks at the environmental variable TZ first
Proceeds to use the localtime library function.
Looks under the shared directory or the system directory for matching timezone information.


So it seems like the 9.1 docs are still accurate as of PostgreSQL 10.

Internally, you can see the list of timezone information by using SELECT * FROM pg_timezone_names(); (here in the source) which calls pg_tzenumerate_start which the reads the shared directory or system directory above.