Written warnings are permanent, and only removable via appeal after 90 days.
Please see 2.1 in general policies "Disciplinary Actions & Appeals" for more information.
https://docs.google.com/document/d/1kFZk_xF8Jiv0QD1lqUfb3sBb8QDVmffscDrPAMnfVQE/edit?pli=1
Here is an excerpt from the document.