Some of the postgres fdws dont pass conditions to the foreign data source because the mapping is sometimes "too hard". So the fdw will do something like select * from table, stream the results and postgres will do the rest of the heavy lifting for you.

Though for the fdws that talk to other SQL based databases, they will pass most of the conditions on to the foreign data source as it is "usually" all just sql.

So the answer is... it depends! :P

edited: I forgot how to write sentences good and stuff