The official Club Penguin help page on the Disney Interactive Support website states that they do indeed remove accounts that are inactive for a certain amount of time, so make sure that if you haven't logged in for some time, don't abandon your penguin. The websites official text says:
Once you've activated your account, make sure you log in every few months so it stays active! Free accounts will be removed if you don't log in for 1 year, and member accounts will be removed after 2 years.
If you created your free account at clubpenguin.com, it will be automatically deleted from our system if the account isn't activated within 10 days after it's created. If you created your account on the Club Penguin app, it will be automatically deleted after a year if it's never been activated.
I'd assume that there is an exception for beta penguins/former moderators and such. However, you must be careful to keep your penguin alive if you decide to quit or venture off into another game. Waddle on!