Howie Hawkins was on the ballot in 2020. He's not on in 2024. That's not because he was removed, it was because he didn't apply.
You don't keep ballot access every election year. You have to apply, present paperwork, and qualify every election to get onto the ballot.
If, for example, a Candidate in 2020 moved out of the country, and changed their permanent address, etc, so that they have not been a US resident for the past 14 years come 2024, their applications to be placed onto the ballot would be rejected, since they fail one of the requirements to be eligible president.