This is a pretty good article that goes over what happens when you archive: https://community.marketo.com/MarketoArticle?id=kA050000000Kzc4CAC. You can use a filter to select archived assets for use in reports.
No, archiving is great because it does not remove items from reports. You can also leave campaigns active.
Marketo does deactivate trigger campaigns that have had no activity in past 180 days.
Re-opening this old thread as I have an Engagement Program which I had turned OFF and Archived, and now cannot get any Email Performance Metrics from it. I have un-archived it, and turned it back on (ensuring to make sure Streams don't have a Cadence set) ,..and still nothing in the email reports.
I'm opening a Support Ticket to investigate if this is just a bug, but curious if there is something I'm missing.