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/usr/share/zsh/help/unlimit is in zsh-common 5.1.1-1ubuntu2.3.

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unlimit [ -hs ] resource ...
       The  resource  limit for each resource is set to the hard limit.
       If the -h flag is given and the  shell  has  appropriate  privi-
       leges,  the  hard  resource  limit for each resource is removed.
       The resources of the shell process are only changed  if  the  -s
       flag is given.

       The  unlimit  command  is not made available by default when the
       shell starts in a mode emulating another shell.  It can be  made
       available with the command `zmodload -F zsh/rlimits b:unlimit'.