Typically, when people start using to-do lists, they start out motivated, but their motivation peters out. This phenomenon is well-documented.
A variety of factors drive this gradual shift. One thing which saps people's motivation: discovering a written record of things they procrastinate on. Procrastination is a form of denial - when you're procrastinating, you're deciding not to do something, but selling yourself the lie that you'll do it later. Reviewing a to-do list then means confronting yourself with a list of tired lies.
In a situation like this, the solution is obvious.
Any type of to-do list system needs periodic review, and that review must include taking out the trash. To-do lists with expired or invalid tasks are like the Lizard of Guilt, and should be thrown out the window. Otherwise, stale hypocrisies cobweb your system, and nothing gets done.
Related: you're not doing Agile if you're not doing retrospectives.