REMOVED OPTIONS / SYSTEM VARIABLES
  * mysqld removed options will not stop the server from starting. They
    will be silently accepted, as if --loose flag is set. Removed options
    prefixes will not interfere with existing options prefixes. They are
    consumed last.
  * mysqld removed options will not show in --help.
  * mysqld system variables will be removed according to the deprecation
    & removal timeline and not function within the client interface once
    removed.

DEPRECTATED OPTIONS / SYSTEM VARIABLES
  * mysqld deprecated options will issue a warning to the user.
  * mysqld deprecated options will still be visible in --help.
  * mysqld system variables will be removed in the next GA version that is
    past EOL of the version that deprecated the variable.
  * deprecated options / variables will not be used anywhere in the server
    code the moment they are deprecated. At most, they will act as aliases.

The advantage of this policy is that it ensures upgrades will always
allow the user to start the server, even when upgrading from a very old
version. It is still possible for user applications to break when
upgrading, as system variables set via the client interface will return
errors. However, this will happen after a long time, with lots of
warnings between versions. The expected timeline is ~ 5 years until a
deprecated variable dissapears from the server.