Best practice to mark deprecated code in Ruby?

For almost all cases, depending on a library or metaprogramming for a deprecation is overkill. Just add a comment to the rdoc and call the Kernel#warn method. For example:

class Foo
  # <b>DEPRECATED:</b> Please use <tt>useful</tt> instead.
  def useless
    warn "[DEPRECATION] `useless` is deprecated.  Please use `useful` instead."
    useful
  end

  def useful
    # ...
  end
end

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If you're using Yard instead of rdoc, your doc comment should look like this:

# @deprecated Please use {#useful} instead

Lastly, if you adhere to tomdoc, make your comment look like this:

# Deprecated: Please use `useful` instead

Deprecated: Indicates that the method is deprecated and will be removed in a future version. You SHOULD use this to document methods that were Public but will be removed at the next major version.

---

Also, don't forget to remove the deprecated method in some future (and properly semver'd) release. Don't make the same mistakes that the Java libraries did.

Ruby Standard Library has a module with the warning logic: https://ruby-doc.org/stdlib/libdoc/rubygems/rdoc/Gem/Deprecate.html. I tend to prefer it to maintain my deprecation messages in a "standard" way:

# my_file.rb

class MyFile
  extend Gem::Deprecate

  def no_more
    close
  end
  deprecate :no_more, :close, 2015, 5

  def close
    # new logic here
  end
end

MyFile.new.no_more
# => NOTE: MyFile#no_more is deprecated; use close instead. It will be removed on or after 2015-05-01.
# => MyFile#no_more called from my_file.rb:16.

Note that with this approach you will gain for free information about where the call took place.

Using ActiveSupport:

class Player < ActiveRecord::Base
  def to_s
    ActiveSupport::Deprecation.warn('Use presenter instead')
    partner_uid
  end
end

Warnings are turned off in production environment by default