deploy.rb

lock '3.11.0'
....
set :unicorn_exec, :unicorn_rails
...
namespace :unicorn do
  before :start, :logging

  task :logging do
    on roles :app do
      execute :echo, fetch(:unicorn_exec)
    end
  end
end

and execute log is below.

sizer@geb:~/capistrano3-unicorn-test
$ bundle exec cap staging unicorn:start
00:00 unicorn:logging
      01 echo unicorn_rails
      01 unicorn_rails
    ✔ 01 centos@staging 0.178s
00:00 unicorn:start
      01 RBENV_ROOT=~/.rbenv/ RBENV_VERSION=2.4.5 ~/.rbenv/bin/rbenv exec bundle exec unicorn -c ./config/unicorn.rb -E staging -D 
    ✔ 01 centos@staging 9.317s

echo returns 'unicorn_rails' correctly, but unicorn:start returns default value... 🤔

execute() has a subtle behaviour. When calling within './directory' { execute(:bundle, :install) } for example, the first argument to execute() is a Stringish with no whitespace. This allows the command to pass through the SSHKit::CommandMap which enables a number of powerful features.
https://capistranorb.com/documentation/getting-started/tasks/

I recommend that removing whitespace from tasks/unicorn.rake .