The best replacement for unless is until.

The best alternatives to unless are rated as following:
★★★★★  until
★★★  if not
★★★  otherwise
★★  not unless
★★  unless
★★  if
★  as long as
★  except
★  provided
★  if exercise
  lest
  without
  inless
  only if
  in case
  if grammar

Your alternative for unless is not listed? Suggest!
Speaking about unless, one probably shouldn't forget about elsif.

Other search terms related to unless and their ratings are:
★★★★★ elsif
★★★★ conditionals
★ first conditional
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unless
until
if not