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If you take an adjective like 長い, the classical conjugation is
長し。(Is long.) (終止形)
長き棒 (long rod) (連体形)

控えたし is 控える+たい. This form is often seen in formal writing, or orders such as this one.

=> 控えたし (we/I want you to show restraint)(=do not...)

The same form is used for the adjective べしin すべし in the next sentence. Another example 至急連絡されたし。