The best replacement for slave is free states.

The best alternatives to slave are rated as following:
★★★★★  free states
★★★★  servant
★★★★  factory worker
★★★  enslaved
★★★  master cylinder
★★★  serf
★★  master hard drive
★★  bondservant
★★  indentured servant
★★  master drive
★  master flash
★  master telephone socket
★  master clutch cylinder
★  master
  cable select
  enslaved person
  client server
  host usb

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

Other search terms related to slave and their ratings are:
★★★★★ master
★ sata
You have in mind a companion for slave and it is not yet here? Propose!



Google Trends

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slave
free states
servant