You are considering archiving or deleting old programs, and you are wondering how that will affect reporting on membership of leads.
If you delete a Marketo Program, the membership data will be lost. Technically, in the Lead record Activity Log, there will not be an activity for "no longer in program," but any and all reporting will not be able to report from deleted programs. Keep in mind that Marketo Support cannot restore deleted programs.
Archiving Programs keeps program membership data. This means that Analytics reporting, including Revenue Explorer, will be able to see archived programs and the related data. However, smart lists will no longer be able to reference the archived program. For example, say the smart list was: "Member of Program, program is [name]". In this situation, the smart list wouldn't be able to locate the program.