Half and half.
An older SQL*Plus client can recognize something starting with SELECT and send it off to the database, so a SELECT CASE.... isn't a problem.

MERGE is liable to confuse it though. It doesn't know MERGE is a new SQL command and throws it back at you the same is if you misspell an SQL*Plus command word.

Also older clients may have problems with newer datatypes (eg TIMESTAMP)

And obviously you don't get the newest SQL*Plus features